Stockholders' Equity - eXponential Stock Performance Plan (Details) $ in Billions	12 Months Ended
	Dec. 31, 2018 USD ($) tranche shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New option granted (in shares) | shares	0	0	0
First tranche	$ 2.5
Incremental increase	$ 1.0
eXponential Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New option granted (in shares) | shares	5,100,000.0
Number of vesting tranches of share-based awards | tranche	12
First tranche	$ 2.5
Incremental increase	$ 1.0