Gains and Losses Recognize in Statement of Consolidated Income for Changes in Level 3 (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]
Total Gains or losses for the period included in earnings	$ (2.3)	[1]	$ 1.6	[2]
Interest Income
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Gains or losses for the period included in earnings	0.1	[1]	4.3	[2]
Investment security gains (losses)
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total Gains or losses for the period included in earnings	$ (1.6)	[1]	$ 0.1	[2]

[1] Changes in unrealized gains or losses included in earnings for assets and liabilities held at period-end include operating expense of $2.3 million. There were no changes in unrealized gains or losses included in earnings for assets or liabilities held at the end of the period that were included in interest income or investment security gains or losses.

[2] Changes in unrealized gains or losses included in earnings for assets and liabilities held at period-end include operating income of $1.6 million. There were no changes in unrealized gains or losses included in earnings for assets or liabilities held at the end of the period that were included in interest income or investment security gains or losses.