Fair Value Measurements - Effect of Interest Rate Swap Contracts Designated as Cash Flow Hedges (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total interest expense which reflects the effects of cash flow hedges		$ (589)	$ (597)	$ (610)
Hedged item		(20)
Derivative designated as hedging instrument		$ 19
Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Total interest expense which reflects the effects of cash flow hedges	$ (597)
Hedged item	(17)
Derivative designated as hedging instrument	$ 14