Equity Capital Requirements (Details) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 capital_requirement	Dec. 31, 2012
Class of Stock [Line Items]
Excess Capital, Core Capital In Excess of Capital Required for Capital Adequacy	$ 179.1	$ 145.0
Excess Capital, Core Capital In Excess of Critical Capital	371.3	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	384.5	374.0
Capital Required for Critical Capital Adequacy	192.3	187.0
Core Capital	563.6	519.0
Risk-based Capital Required for Regulatory Capital Adequacy	123.6	58.1
Regulatory Capital	577.1	535.9
Excess Capital, Regulatory Capital In Excess of Risk-Based Capital	$ 453.5	$ 477.8