Stockholders' Equity - Fair Value of Awards Granted and Vested (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of awards granted	$ 37	$ 17
Total fair value of awards vested	15	16
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of awards granted	23	17
Total fair value of awards vested	12	10
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of awards granted	14	0
Performance Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of awards vested	$ 3	$ 6