Employee Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Amounts recognized in balance sheet
Regulatory assets	$ 216.5	$ 199.7
Accrued benefit costs and other current liabilities	(5.3)	(2.9)
Deferred credits and other liabilities	(180.6)	(175.9)
Accumulated other comprehensive loss (income) (pretax)	53.1	50.8
Net amount recognized at end of year	83.7	71.7
Other Postretirement Benefits [Member]
Amounts recognized in balance sheet
Regulatory assets	59.6	52.7
Accrued benefit costs and other current liabilities	(13.1)	(13.2)
Deferred credits and other liabilities	(217.2)	(203.3)
Accumulated other comprehensive loss (income) (pretax)	(17.9)	(20.4)
Net amount recognized at end of year	$ (188.6)	$ (184.2)