Pension Benefit and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,150		$ 1,060	$ 3,447	$ 3,180
Interest cost	2,094		2,116	6,336	6,348
Expected return on plan assets	(2,495)		(2,591)	(7,397)	(7,773)
Amortization of prior service cost	116		150	347	450
Amortization of net loss	1,347		553	4,138	1,659
Special termination benefit
Settlement charge	2,023			2,023
Net periodic benefit cost	4,235		1,288	8,894	3,864
Retirement Health and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	149		171	481	523
Interest cost	92		110	272	304
Expected return on plan assets
Amortization of prior service cost	(113)		(157)	(338)	(471)
Amortization of net loss	66		130	247	292
Special termination benefit		1,600		1,593
Settlement charge
Net periodic benefit cost	$ 194		$ 254	$ 2,255	$ 648