Equity Changes in AOCI (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ (3,686)	$ 7,500		$ (3,686)
Total other comprehensive income (loss)		(36)	$ 25	(357)	$ (52)
Ending balance		6,975		6,975		$ (3,686)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%					35.00%
Accumulated net unrealized investment gain (loss), AFS securities [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 510	304	643	510	716	$ 716
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act		0	0	110	0
Other comprehensive income (loss) before reclassifications		(31)	47	(84)	67
Amounts reclassified from other comprehensive income (loss)		(2)	(20)	(265)	(113)
Total other comprehensive income (loss)		(33)	27	(349)	(46)
Ending balance		271	670	271	670	510
Accumulated other comprehensive income (loss), Other [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	43	45	39	43	43	43
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act		0	0	7	0
Other comprehensive income (loss) before reclassifications		0	0	0	0
Amounts reclassified from other comprehensive income (loss)		(3)	(2)	(8)	(6)
Total other comprehensive income (loss)		(3)	(2)	(8)	(6)
Ending balance		42	37	42	37	43
Accumulated other comprehensive income [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 553	349	682	553	759	759
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act		0	0	117	0
Other comprehensive income (loss) before reclassifications		(31)	47	(84)	67
Amounts reclassified from other comprehensive income (loss)		(5)	(22)	(273)	(119)
Total other comprehensive income (loss)		(36)	25	(357)	(52)
Ending balance		$ 313	$ 707	$ 313	$ 707	$ 553