Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 153	$ 83	$ 281	$ 210
Other comprehensive income:
Gains (losses) on derivatives	(162)	42	(253)	173
Reclassification adjustments for derivative (gains) losses included in net income (interest expense)	(2)	(1)	(3)	(1)
Total gains (losses) on derivatives	(164)	41	(256)	172
Income tax (expense) benefit	41	(11)	63	(43)
Other comprehensive income (loss), net of tax	(123)	30	(193)	129
Total comprehensive income	$ 30	$ 113	$ 88	$ 339