Equity - Capital Requirements (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Regulatory Capital Requirements [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	$ 442.8	$ 421.3
Core Capital	558.2	766.3
Excess Capital, Core Capital In Excess of Capital Required for Capital Adequacy	$ 115.4	$ 345.0