Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Defined Benefit Plan Disclosure [Line Items]
Amortization of transition obligation		$ 8
Amortization of prior year service cost (benefit)	(16)	(12)
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	12,466	11,588	12,334
Interest cost	32,768	31,671	32,612
Expected return on plan assets	(31,849)	(30,910)	(30,286)
Amortization of transition obligation		13	25
Amortization of prior year service cost (benefit)		7	23
Recognized net actuarial (gain) loss	3,887	1,674	82
Settlement gain	23	(8)	(9)
Net periodic benefit cost	17,295	14,035	14,781
Post-retirement
Defined Benefit Plan Disclosure [Line Items]
Service cost	180	200	214
Interest cost	1,889	2,013	2,377
Amortization of prior year service cost (benefit)	(27)	(27)	(41)
Recognized net actuarial (gain) loss	(3,306)	(3,295)	(3,326)
Net periodic benefit cost	$ (1,264)	$ (1,109)	$ (776)