Accumulated Other Comprehensive Income - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Realized investment (gains) losses	$ 3	$ (51)	$ 36	$ (97)
Other operating expenses	2,893	2,412	5,695	5,067
Securities [ Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Realized investment (gains) losses	(2)	(15)	28	(50)
Derivatives [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Realized investment (gains) losses	0	0	0	(4)
Reclassification adjustment for amortization of net losses from past experience and prior service costs [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other operating expenses	17	17	34	32
Reclassification adjustment for settlement losses [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other operating expenses	$ 0	$ 0	$ 0	$ 6