Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Special termination benefit	$ 2,300
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,189	1,060
Interest cost	2,118	2,116
Expected return on plan assets	(2,408)	(2,591)
Amortization of prior service cost	116	150
Amortization of net loss	1,349	553
Special benefits acceleration
Net periodic benefit cost	2,364	1,288
Retirement Health and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	182	176
Interest cost	88	97
Amortization of prior service cost	(113)	(157)
Amortization of net loss	115	81
Special benefits acceleration
Special termination benefit	2,300
Net periodic benefit cost	$ 2,572	$ 197