Equity Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ (3,686)
Total other comprehensive income (loss)	(323)	$ 6
Ending balance	$ 3,938		$ (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	716	$ 716
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act(2)	110	0
Other comprehensive income (loss) before reclassifications	(57)	9
Amounts reclassified from other comprehensive income (loss)	(263)	(1)
Total other comprehensive income (loss)	(320)	8
Ending balance	300	724	510
Accumulated other comprehensive income (loss), Other [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	43	43	43
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act(2)	7	0
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from other comprehensive income (loss)	(3)	(2)
Total other comprehensive income (loss)	(3)	(2)
Ending balance	47	41	43
Accumulated other comprehensive income [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	553	759	759
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act(2)	117	0
Other comprehensive income (loss) before reclassifications	(57)	9
Amounts reclassified from other comprehensive income (loss)	(266)	(3)
Total other comprehensive income (loss)	(323)	6
Ending balance	$ 347	$ 765	$ 553