Goodwill and Other Intangible Assets - Gross Carrying Amount, Accumulated Amortization And Net Carrying Amount Of Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 3,520	$ 3,497
Accumulated Amortization	(1,942)	(1,884)
Net Carrying Amount	1,578	1,613	$ 1,750
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	2,686	2,669
Accumulated Amortization	(1,515)	(1,470)
Net Carrying Amount	1,171	1,199
Core deposits
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	692	686
Accumulated Amortization	(332)	(320)
Net Carrying Amount	360	366
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	142	142
Accumulated Amortization	(95)	(94)
Net Carrying Amount	$ 47	$ 48