Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Preferred Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Balance at beginning of period at Dec. 31, 2017	$ 10,294	$ 0	$ 0	$ 7,525	$ (768)	$ 3,537
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of accounting standards	40					40
Net income	369					369
Other comprehensive income (loss)	60				60
Stock based compensation	16			16
Dividends paid	(30)					(30)
Other	(1)					(1)
Balance at end of period at Mar. 31, 2018	10,748	0	0	7,541	(708)	3,915
Balance at beginning of period at Dec. 31, 2017	10,294	0	0	7,525	(768)	3,537
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,252
Balance at end of period at Sep. 30, 2018	11,852	0	0	8,052	(970)	4,770
Balance at beginning of period at Mar. 31, 2018	10,748	0	0	7,541	(708)	3,915
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	442					442
Other comprehensive income (loss)	(228)				(228)
Stock based compensation	13			13
Other	1					1
Balance at end of period at Jun. 30, 2018	10,976	0	0	7,554	(936)	4,358
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	441					441
Other comprehensive income (loss)	(34)				(34)
Stock based compensation	6			6
Issuance of preferred stock	492			492
Dividends paid	(29)					(29)
Balance at end of period at Sep. 30, 2018	11,852	0	0	8,052	(970)	4,770
Balance at beginning of period at Dec. 31, 2018	11,659	0	0	8,058	(1,066)	4,667
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	271					271
Other comprehensive income (loss)	42				42
Stock based compensation	11			11
Other	1					1
Balance at end of period at Mar. 31, 2019	11,984	0	0	8,069	(1,024)	4,939
Balance at beginning of period at Dec. 31, 2018	11,659	0	0	8,058	(1,066)	4,667
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,190
Balance at end of period at Sep. 30, 2019	12,688	0	0	8,085	(1,210)	5,813
Balance at beginning of period at Mar. 31, 2019	11,984	0	0	8,069	(1,024)	4,939
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	403					403
Other comprehensive income (loss)	(10)				(10)
Stock based compensation	8			8
Balance at end of period at Jun. 30, 2019	12,385	0	0	8,077	(1,034)	5,342
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	516					516
Other comprehensive income (loss)	(176)				(176)
Stock based compensation	8			8
Dividends paid	(45)					(45)
Balance at end of period at Sep. 30, 2019	$ 12,688	$ 0	$ 0	$ 8,085	$ (1,210)	$ 5,813