Equity - Capital Requirements (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Equity [Abstract]
Capital Requirements, Minimum Capital, Percentage of Aggregate On-Balance Sheet Assets Included In Core Capital	2.75%
Capital Requirements, Minimum Capital, Percentage of Aggregate Off-Balance Sheet Assets Included In Core Capital	0.75%
Capital Requirements, Critical Capital, Percentage of Core Capital of Total Minimum Capital	50.00%
Capital Required for Capital Adequacy	$ 515.8	$ 466.5
Capital	653.4	609.7
Excess Capital	$ 137.6	$ 143.2