Equity - Capital Requirements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
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	$ 403.0	$ 398.5
Capital Required for Critical Capital Adequacy	201.5	199.3
Core Capital	664.0	590.7
Excess Capital, Core Capital In Excess of Capital Required for Capital Adequacy	261.0	192.2
Excess Capital, Core Capital In Excess of Critical Capital	462.5	391.4
Risk-based Capital Required for Regulatory Capital Adequacy	90.4	90.8
Regulatory Capital	678.0	604.0
Excess Capital, Regulatory Capital In Excess of Risk-Based Capital	$ 587.6	$ 513.2