Goodwill (Changes in the Net Carrying Amount of Goodwill by Reporting Segment) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill
Goodwill				$ 3,381
Accumulated impairment losses			$ (965)
Goodwill Roll Forward
Net carrying value, beginning of period:	$ 2,445	$ 2,416
Acquisition activity	17	7
Net carrying value, end of period:	2,462	2,423
Commercial Banking
Goodwill
Goodwill				1,363
Accumulated impairment losses			(750)
Goodwill Roll Forward
Net carrying value, beginning of period:	613	613
Acquisition activity	17	0
Net carrying value, end of period:	630	613
Branch Banking
Goodwill
Goodwill				1,655
Accumulated impairment losses			0
Goodwill Roll Forward
Net carrying value, beginning of period:	1,655	1,655
Acquisition activity	0	0
Net carrying value, end of period:	1,655	1,655
Consumer Lending
Goodwill
Goodwill				215
Accumulated impairment losses			(215)
Goodwill Roll Forward
Net carrying value, beginning of period:	0	0
Acquisition activity	0	0
Net carrying value, end of period:	0	0
Wealth and Asset Management
Goodwill
Goodwill				$ 148
Accumulated impairment losses			$ 0
Goodwill Roll Forward
Net carrying value, beginning of period:	177	148
Acquisition activity	0	7
Net carrying value, end of period:	$ 177	$ 155