Income Taxes - Schedule of Income Taxes (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Income Tax Expense Benefit Continuing Operations Income Tax Reconciliation [Line Items]
Expected income tax expense, at statutory rate	$ 3,811	$ 4,092
U.S. state income tax expense, net	497	507
Other, net	(83)	(175)
Income tax expense	4,225	4,424
Comprehensive provision for income taxes allocable to:
Net income	4,225	4,424
Income tax expense	5,120	5,465
Defined Benefit Pension Plans [Member]
Comprehensive provision for income taxes allocable to:
Other comprehensive income (loss) Pension plans	1,760	1,882
OPEB [Member]
Comprehensive provision for income taxes allocable to:
Other comprehensive income (loss) Pension plans	$ (865)	$ (841)