Consolidated Statements of Shareholder's Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Beginning Balance at Dec. 31, 2017	$ 1,871	$ 0	$ 161	$ (998)	$ 2,708
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	326				326
Other comprehensive income (loss)	(27)			(27)	0
Ending Balance at Sep. 30, 2018	2,170	0	161	(1,025)	3,034
Beginning Balance at Jun. 30, 2018	2,077	0	161	(1,009)	2,925
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	109				109
Other comprehensive income (loss)	(16)			(16)	0
Ending Balance at Sep. 30, 2018	2,170	0	161	(1,025)	3,034
Beginning Balance at Dec. 31, 2018	2,196	0	161	(1,060)	3,095
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	325				325
Other comprehensive income (loss)	(39)			(39)	0
Ending Balance at Sep. 30, 2019	2,482	0	161	(1,099)	3,420
Beginning Balance at Jun. 30, 2019	2,396	0	161	(1,075)	3,310
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	110				110
Other comprehensive income (loss)	(24)			(24)	0
Ending Balance at Sep. 30, 2019	$ 2,482	$ 0	$ 161	$ (1,099)	$ 3,420