Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Line Items]
Valuation allowance		$ 1
Credit carryforward	96	226
Total deferred tax asset	1,001	1,104
Unrecognized tax benefits	6	6	7
Net interest credits (expenses)	10.6	1.4	0.2
Recovery of penalties related to unrecognized tax benefits in income tax expense	0	0.2	0
Accrued interest payable	1.2	1.1
Reduction in federal tax credit carryforward	1
Maximum [Member]
Income Taxes [Line Items]
Valuation allowance	1
Federal [Member]
Income Taxes [Line Items]
Credit carryforward	96
State [Member]
Income Taxes [Line Items]
Credit carryforward	6
State net operating loss carryforwards	62
Total deferred tax asset	9
Accrued state tax penalties	$ 0.3	$ 0.3