Loans Payable - Outstanding Borrowing which Includes Preferred Shares (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	$ 14,577.8	$ 10,718.4
Fair Value	13,656.7	9,689.9
Senior Secured Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	13,662.3	10,291.2
Fair Value	12,658.4	9,010.7
Weighted Average Interest Rate	1.30%	1.44%
Weighted Average Remaining Maturity in Years	8 years 9 months 18 days	8 years 10 months 6 days
Subordinated Notes Income Notes And Preferred Shares [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	914.8	417.3
Fair Value	996.9	670.7
Weighted Average Remaining Maturity in Years	8 years 2 months 19 days	8 years 6 months 15 days
Combination Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	0.7	9.9
Fair Value	$ 1.4	$ 8.5
Weighted Average Remaining Maturity in Years	8 years 9 months 22 days	9 years 11 months 1 day