Changes in and Reclassifications From Accumulated Other Comprehensive Income (Loss) - AOCI Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 16,903	$ 17,642
OCI before reclassifications	731	(13)	$ (1,279)
Amounts reclassified from AOCI	269	5	22
OCI, net of tax	1,000	(8)	(1,257)
Ending balance	13,494	16,903	17,642
Net Unrealized Gain on Securities
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1,276	1,279	2,370
OCI before reclassifications	857	83	(1,112)
Amounts reclassified from AOCI	(202)	(86)	21
OCI, net of tax	655	(3)	(1,091)
Ending balance	1,931	1,276	1,279
OTTI Losses in OCI
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(3)	(7)	(5)
OCI before reclassifications	0	1	(3)
Amounts reclassified from AOCI	0	3	1
OCI, net of tax	0	4	(2)
Ending balance	(3)	(3)	(7)
Net Gain on Cash Flow Hedging Instruments
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	76	130	150
OCI before reclassifications	(8)	(8)	18
Amounts reclassified from AOCI	(50)	(46)	(38)
OCI, net of tax	(58)	(54)	(20)
Ending balance	18	76	130
Foreign Currency Translation Adjustments
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	6	(55)	(8)
OCI before reclassifications	28	(37)	(47)
Amounts reclassified from AOCI	0	98	0
OCI, net of tax	28	61	(47)
Ending balance	34	6	(55)
Pension and Other Postretirement Plan Adjustments
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(1,692)	(1,676)	(1,579)
OCI before reclassifications	(146)	(52)	(135)
Amounts reclassified from AOCI	521	36	38
OCI, net of tax	375	(16)	(97)
Ending balance	(1,317)	(1,692)	(1,676)
AOCI, net of tax
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(337)	(329)	928
Ending balance	$ 663	$ (337)	$ (329)