Equity - Capital Requirements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Capital Requirements [Abstract]
Capital Requirements, Minimum Capital, Percentage of Aggregate On-Balance Sheet Assets Included In Core Capital	2.75%
Excess Capital, Core Capital In Excess of Capital Required for Capital Adequacy	$ 345.0	$ 192.2
Excess Capital, Core Capital In Excess of Critical Capital	555.6	391.4
Capital Required for Capital Adequacy	421.3	398.5
Capital Required for Critical Capital Adequacy	210.7	199.3
Core Capital	766.3	590.7
Risk-based Capital Required for Regulatory Capital Adequacy	121.6	90.8
Regulatory Capital	776.4	604.0
Excess Capital, Regulatory Capital In Excess of Risk-Based Capital	$ 654.8	$ 513.2
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%