Assets and Liabilities Measured at Fair Value on a Recurring Basis Segregated by Fair Value Hierarchy Level (Parenthetical) (Detail) (Fair Value, Measurements, Recurring, Netting and Collateral, USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Derivative Financial Instruments, Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	$ 62.9	$ 2,631.7
Derivative Financial Instruments, Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	$ (1,564.1)	$ (2,952.7)