Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning			$ (1,082)
Shareholders' other comprehensive income (loss), net of tax	$ (296)	$ 106	(522)	$ 236
Accumulated other comprehensive income (loss), ending	(1,843)		(1,843)
Securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	110	369	389	362
Other comprehensive income (loss) before reclassifications, pre-tax	(176)	115	(555)	168
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	37	(38)	113	(62)
Other comprehensive income (loss) before reclassifications, after-tax	(139)	77	(442)	106
Reclassification adjustment, pre-tax	(2)	(15)	28	(50)
Reclassification adjustment, tax (expense) benefit	0	5	(6)	18
Reclassification adjustment, after-tax	(2)	(10)	22	(32)
Shareholders' other comprehensive income (loss), net of tax	(141)	67	(420)	74
Accumulated other comprehensive income (loss), ending	(31)	436	(31)	436
Securities [ Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	110	369	328	362
Securities [ Member] | Restatement Adjustment [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	0	0	65	0
Securities [ Member] | Restatement Adjustment [Member] | Accounting Standards Update 2016-01 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	0	0	(4)	0
Derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(11)	0	(6)	3
Other comprehensive income (loss) before reclassifications, pre-tax	(10)	0	(16)	0
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	2	0	3	0
Other comprehensive income (loss) before reclassifications, after-tax	(8)	0	(13)	0
Reclassification adjustment, pre-tax	0	0	0	(4)
Reclassification adjustment, tax (expense) benefit	0	0	0	1
Reclassification adjustment, after-tax	0	0	0	(3)
Shareholders' other comprehensive income (loss), net of tax	(8)	0	(13)	(3)
Accumulated other comprehensive income (loss), ending	(19)	0	(19)	0
Derivatives [Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(11)	0	0	3
Derivatives [Member] | Restatement Adjustment [Member] | Accounting Standards Update 2017-12 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	0	0	(6)	0
Translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(24)	(257)	(69)	(369)
Other comprehensive income (loss) before reclassifications, pre-tax	(151)	30	(105)	141
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	(1)	(3)	(2)	(2)
Other comprehensive income (loss) before reclassifications, after-tax	(152)	27	(107)	139
Accumulated other comprehensive income (loss), ending	(176)	(230)	(176)	(230)
Translation of foreign currencies [Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(24)	(257)	(65)	(369)
Translation of foreign currencies [Member] | Restatement Adjustment [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	0	0	(4)	0
Postretirement benefits liability [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(1,622)	(1,364)	(1,635)	(1,378)
Other comprehensive income (loss) before reclassifications, pre-tax	(12)	2	(12)	2
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	3	0	3	0
Other comprehensive income (loss) before reclassifications, after-tax	(9)	2	(9)	2
Reclassification adjustment, tax (expense) benefit	(3)	(7)	(7)	(14)
Reclassification adjustment, after-tax	14	10	27	24
Shareholders' other comprehensive income (loss), net of tax	5	12	18	26
Accumulated other comprehensive income (loss), ending	(1,617)	(1,352)	(1,617)	(1,352)
Postretirement benefits liability [Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(1,622)	(1,364)	(1,345)	(1,378)
Postretirement benefits liability [Member] | Restatement Adjustment [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	0	0	(290)	0
Reclassification adjustment for amortization of net losses from past experience and prior service costs [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	17	17	34	32
Reclassification adjustment for settlement losses [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	$ 0	$ 0	$ 0	$ 6