Equity - Capital Requirements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 capital_requirement	Dec. 31, 2011
Equity [Abstract]
Number of capital requirements	3
Percentage of aggeregate on-balance sheet assets included in core capital	2.75%
Percentage of aggeregate off-balance sheet assets included in core capital	0.75%
Critical Capital, percentage of core capital of total minimum capital	50.00%
Minimum capital requirement	$ 374.0	$ 348.7
Critical capital requirement	187.0	174.3
Core capital	519.0	475.2
Core capital in excess of capital required for capital adequacy	145.0	126.5
Core capital in excess of critical capital	332.0	300.9
Risk-based capital required for regulatory capital adequacy	58.1	52.9
Regulatory capital	535.9	492.7
Regulatory capital in excess of risk-based capital	$ 477.8	$ 439.8