Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	$ (1,082)
Shareholders' other comprehensive income (loss), net of tax	(226)	$ 130
Accumulated other comprehensive income (loss), ending	(1,547)
Securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	389	362
Other comprehensive income (loss) before reclassifications, pre-tax	(379)	53
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	76	(24)
Other comprehensive income (loss) before reclassifications, after-tax	(303)	29
Reclassification adjustment, pre-tax	30	(35)
Reclassification adjustment, tax (expense) benefit	(6)	13
Reclassification adjustment, after-tax	24	(22)
Shareholders' other comprehensive income (loss), net of tax	(279)	7
Accumulated other comprehensive income (loss), ending	110	369
Securities [ Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	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	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	(4)	0
Derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(6)	3
Other comprehensive income (loss) before reclassifications, pre-tax	(6)	0
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	1	0
Other comprehensive income (loss) before reclassifications, after-tax	(5)	0
Reclassification adjustment, pre-tax	0	(4)
Reclassification adjustment, tax (expense) benefit	0	1
Reclassification adjustment, after-tax	0	(3)
Shareholders' other comprehensive income (loss), net of tax	(5)	(3)
Accumulated other comprehensive income (loss), ending	(11)	0
Derivatives [Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	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	(6)	0
Translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(69)	(369)
Other comprehensive income (loss) before reclassifications, pre-tax	46	111
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	(1)	1
Other comprehensive income (loss) before reclassifications, after-tax	45	112
Accumulated other comprehensive income (loss), ending	(24)	(257)
Translation of foreign currencies [Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(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	(4)	0
Postretirement benefits liability [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(1,635)	(1,378)
Reclassification adjustment, tax (expense) benefit	(4)	(7)
Shareholders' other comprehensive income (loss), net of tax	13	14
Accumulated other comprehensive income (loss), ending	(1,622)	(1,364)
Postretirement benefits liability [Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(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	(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	15
Reclassification adjustment for settlement losses [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	$ 0	$ 6