Gains and Losses Recognize in Statement of Consolidated Income for Changes in Level 3 (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Operating Income (Expense)
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in unrealized gains or losses included in earnings for assets and liabilities	$ 2.3	$ 1.6
Interest Income
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in unrealized gains or losses included in earnings for assets and liabilities	0	0
Investment security gains (losses)
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in unrealized gains or losses included in earnings for assets and liabilities	$ 0	$ 0