Equity Capital Requirements (Details) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 capital_requirement	Dec. 31, 2012
Class of Stock [Line Items]
Excess Capital, Core Capital In Excess of Capital Required for Capital Adequacy	$ 155.6	$ 145.0
Excess Capital, Core Capital In Excess of Critical Capital	346.0	332.0
Number of Capital Requirements	3
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	380.9	374.0
Capital Required for Critical Capital Adequacy	190.5	187.0
Core Capital	536.5	519.0
Risk-based Capital Required for Regulatory Capital Adequacy	66.5	58.1
Regulatory Capital	550.7	535.9
Excess Capital, Regulatory Capital In Excess of Risk-Based Capital	$ 484.2	$ 477.8