Goodwill and Other Intangible Assets - Gross Carrying Amount, Accumulated Amortization And Net Carrying Amount Of Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 3,478	$ 3,413
Accumulated Amortization	(1,820)	(1,663)
Net Carrying Amount	1,658	1,750	$ 1,768
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	2,654	2,620
Accumulated Amortization	(1,418)	(1,306)
Net Carrying Amount	1,236	1,314
Core deposits
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	683	661
Accumulated Amortization	(311)	(277)
Net Carrying Amount	372	384
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	141	132
Accumulated Amortization	(91)	(80)
Net Carrying Amount	$ 50	$ 52