Investment Securities (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Debt securities transferred from AFS to HTM	$ 24,700
Unrealized loss on transferred securities	227
Consolidated assets	230,714		$ 223,383
Fair value of pledged securities	$ 936
Scenario, Forecast [Member]
Schedule of Held-to-maturity Securities [Line Items]
Consolidated assets		$ 250,000