The surprising 'prenups' of America's richest couples

• A prenuptial agreement (or prenup) is a legal agreement couples enter into before they get married.
• Many involve what will happen to the couple's finances should they divorce.
• However, relationship contracts, whether they be a prenup, postnup, or other agreement, can stipulate more than simply who gets what in case of a breakup.

Successful power couples agree upon important issues from the get go.

And whether it's with a legally-binding prenup, or postnup, or a more flexible relationship agreement, they often put it in writing.

"Two egos in a marriage can be great if all the important issues are agreed upon up front," Handel Group co-president and life coach Laurie Gerber previously told Business Insider.

While prenups are traditionally thought of as worst-case-scenario financial planning, relationship agreements aren't always all about the money.

Some of the most important issues couples should agree upon early on include sexual and romantic needs, family boundaries, and who pays for what, and putting it into writing can give a marriage a better chance of success.

Most breakups happen because of cheating, Gerber says: "Couples who face the reality of this threat head-on and deal with it stand the most chance of success."

"Don't be afraid to put it all in writing to refer back to as a living breathing document," she says. "And plan to have meetings about how you are running your marriage and family just as you would for your company or any other project you care about profoundly."

Here are some of the less conventional things successful power couples have put into writing:

SEE ALSO: Successful power couples that stay together have 8 things in common

DON'T MISS: Why successful people get divorced

Mark Zuckerberg and Priscilla Chan agreed to spend more time together

Before marrying in 2012, when Priscilla Chan moved to Palo Alto, California, she and Facebook CEO and founder Mark Zuckerberg drew up a relationship agreement.

In it, Zuckerberg agreed to take her on a date once a week and spend 100 minutes of alone time each week with her outside the office or his apartment.

The couple are worth an estimated $74.5 billion.

Jessica Biel and Justin Timberlake have a clause about cheating

Actors Jessica Biel and Justin Timberlake reportedly have an infidelity clause in their prenup that states Biel will receive $500,000 if Timberlake cheats.

The couple are worth an estimated $248 million. 

Nicole Kidman and Keith Urban agreed not to tolerate drug or alcohol abuse

Actress Nicole Kidman and singer songwriter Keith Urban reportedly signed a prenup stipulating that Urban, who struggled with alcoholism and cocaine abuse, would receive $600,000 a year for every year they are together, but only if Urban refrained from using illegal narcotics or drinks excessively.

The couple married in June 2006, and in October 2006 Urban checked into rehab for an alcohol addiction.

"Definitely we both met each other exactly at the right time," Urban told People magazine in 2007. "She said early on that she wanted to be brave with me. I feel there was something else at work, bringing us together and then just continuing to watch over us." 

The couple are worth an estimated $205 million.