Pension Benefit and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,254	$ 1,189
Interest cost	2,061	2,118
Expected return on plan assets	(2,716)	(2,408)
Amortization of prior service cost	93	116
Amortization of net loss	1,476	1,349
Special termination benefit
Net periodic benefit cost	2,168	2,364
Retirement Health and Life Insurance Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	170	182
Interest cost	71	88
Expected return on plan assets
Amortization of prior service cost	(58)	(113)
Amortization of net loss	83	115
Special termination benefit		2,300
Net periodic benefit cost	$ 266	$ 2,572