Equity - Capital Requirements (Details) (USD $) In Millions, unless otherwise specified	Sep. 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	$ 332.9	$ 192.2
Excess Capital, Core Capital In Excess of Critical Capital	547.1	391.4
Capital Required for Capital Adequacy	428.4	398.5
Capital Required for Critical Capital Adequacy	214.2	199.3
Core Capital	761.3	590.7
Risk-based Capital Required for Regulatory Capital Adequacy	67.3	90.8
Regulatory Capital	771.9	604.0
Excess Capital, Regulatory Capital In Excess of Risk-Based Capital	$ 704.6	$ 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%