Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statement of Comprehensive Income
Net income	$ 551	$ 293	$ 595	$ 617	$ 943	$ 716	$ 664	$ 691	$ 2,056	$ 3,014	$ 3,439
Other comprehensive income (loss):
Changes in net unrealized gains on investment securities having no credit losses recognized in the consolidated statement of income									294	(883)	(1,020)
Changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income									8	21	(14)
Net changes in benefit plan assets and obligations									29	16	66
Net changes in unrealized foreign currency translation									191	(41)	(461)
Other comprehensive income (loss) before income taxes									522	(887)	(1,429)
Income tax expense (benefit)									110	(289)	(392)
Other comprehensive income (loss), net of taxes									412	(598)	(1,037)
Comprehensive income									$ 2,468	$ 2,416	$ 2,402