Stock-based Compensation - Nonvested Performance Share Units (Details) - Performance Share Unit Awards [Member] $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Granted in 2013 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value accrued	$ 18
Maximum aggregate payout	24
Granted in 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value accrued	13
Maximum aggregate payout	28
Granted In 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value accrued	7
Maximum aggregate payout	$ 29