Equity Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 3,900		$ (3,686)
Total other comprehensive income (loss)	2	$ (83)	(321)	$ (77)
Ending balance	7,459		$ 7,459		$ (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	300	724	$ 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	4	11	(53)	20
Amounts reclassified from other comprehensive income (loss)	0	(92)	(263)	(93)
Total other comprehensive income (loss)	4	(81)	(316)	(73)
Ending balance	304	643	304	643	510
Accumulated other comprehensive income (loss), Other [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	47	41	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)	(2)	(2)	(5)	(4)
Total other comprehensive income (loss)	(2)	(2)	(5)	(4)
Ending balance	45	39	45	39	43
Accumulated other comprehensive income [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	347	765	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	4	11	(53)	20
Amounts reclassified from other comprehensive income (loss)	(2)	(94)	(268)	(97)
Total other comprehensive income (loss)	2	(83)	(321)	(77)
Ending balance	$ 349	$ 682	$ 349	$ 682	$ 553