Assets and Liabilities Measured at Fair Value on Recurring Basis Segregated by Fair Value Hierarchy Level (Parenthetical) (Detail) (Subject to master netting arrangement or similar arrangement, Fair Value, Measurements, Recurring, USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative Financial Instruments, Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	$ 1,200.0	$ 767.7
Derivative Financial Instruments, Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	$ 315.8	$ 210.7