Goodwill and Other Intangible Assets - Gross Carrying Amount, Accumulated Amortization And Net Carrying Amount Of Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 4,320	$ 4,430
Accumulated Amortization	(2,112)	(2,061)
Net Carrying Amount	2,208	2,369	$ 1,613
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	3,144	3,262
Accumulated Amortization	(1,639)	(1,605)
Net Carrying Amount	1,505	1,657
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	403	389
Accumulated Amortization	(60)	(49)
Net Carrying Amount	343	340
Core deposits
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	673	676
Accumulated Amortization	(356)	(350)
Net Carrying Amount	317	326
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	100	103
Accumulated Amortization	(57)	(57)
Net Carrying Amount	$ 43	$ 46