Equity - Capital Requirements (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 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	$ 368.4	$ 348.7
Critical capital requirement	184.2	174.3
Core capital	508.5	475.2
Core capital in excess of capital required for capital adequacy	140.1	126.5
Core capital in excess of critical capital	324.3	300.9
Risk-based capital required for regulatory capital adequacy	42.0	52.9
Regulatory capital	526.3	492.7
Regulatory capital in excess of risk-based capital	$ 484.3	$ 439.8