Equity Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 6,240
Total other comprehensive loss	(39)	$ (323)
Ending balance	$ 5,361		$ 6,240
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	$ 276	510	$ 510
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act	0	110
Other comprehensive income (loss) before reclassifications	8	(57)
Amounts reclassified from other comprehensive loss	(44)	(263)
Total other comprehensive loss	(36)	(320)
Ending balance	240	300	276
Accumulated other comprehensive income (loss), Other [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	46	43	43
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act	0	7
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from other comprehensive loss	(3)	(3)
Total other comprehensive loss	(3)	(3)
Ending balance	43	47	46
Accumulated Other Comprehensive Income
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	322	553	553
Reclassification of accumulated other comprehensive income to retained earnings resulting from the enactment of the Tax Cuts and Jobs Act	0	117
Other comprehensive income (loss) before reclassifications	8	(57)
Amounts reclassified from other comprehensive loss	(47)	(266)
Total other comprehensive loss	(39)	(323)
Ending balance	$ 283	$ 347	$ 322