Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,108		$ 1,060	$ 2,297	$ 2,120
Interest cost	2,124		2,116	4,242	4,232
Expected return on plan assets	(2,495)		(2,591)	(4,903)	(5,182)
Amortization of prior service cost	116		150	232	300
Amortization of net loss	1,442		553	2,791	1,106
Net periodic benefit cost	2,295		1,288	4,659	2,576
Retirement Health and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	149		176	331	352
Interest cost	92		97	180	194
Amortization of prior service cost	(113)		(157)	(226)	(314)
Amortization of net loss	66		81	181	162
Special termination benefit		1,600		2,300
Adjustment to termination benefit	(707)			(707)
Net periodic benefit cost	$ (513)		$ 197	$ 2,059	$ 394