Equity - Capital Requirements (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 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	$ 317.7	$ 192.2
Excess Capital, Core Capital In Excess of Critical Capital	534.0	391.4
Capital Required for Capital Adequacy	432.6	398.5
Capital Required for Critical Capital Adequacy	216.3	199.3
Core Capital	750.3	590.7
Risk-based Capital Required for Regulatory Capital Adequacy	122.8	90.8
Regulatory Capital	761.7	604.0
Excess Capital, Regulatory Capital In Excess of Risk-Based Capital	$ 638.9	$ 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%