Fair Value Measurements (Details 6) (Nonrecurring Fair Value Measurements, Fair value hierarchy Level 3, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Mortgage loans
Assets and liabilities measured at fair value on a nonrecurring basis
Fair value of assets measured on nonrecurring basis	$ 126.5	$ 49.7
Net (gain) loss due to change in fair value of assets measured on nonrecurring basis	7.9	(0.8)
Mortgage loans | Minimum input
Assets and liabilities measured at fair value on a nonrecurring basis
Discount rate (as a percent)	8.00%
Terminal capitalization rate (as a percent)	6.30%
Average market rent growth rate (as a percent)	8.00%
Mortgage loans | Maximum input
Assets and liabilities measured at fair value on a nonrecurring basis
Discount rate (as a percent)	13.00%
Terminal capitalization rate (as a percent)	10.30%
Average market rent growth rate (as a percent)	13.00%
Mortgage servicing rights
Assets and liabilities measured at fair value on a nonrecurring basis
Fair value of assets measured on nonrecurring basis	4.4	1.1
Net (gain) loss due to change in fair value of assets measured on nonrecurring basis	$ (0.1)	$ (0.1)
Mortgage servicing rights | Input
Assets and liabilities measured at fair value on a nonrecurring basis
Discount rate (as a percent)	3.30%