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Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 15,562 $ 13,529 $ 15,834
Interest cost 28,066 29,757 26,791
Expected return on plan assets (62,726) (57,550) (52,480)
Amortization of prior service cost (credit) 1,037 1,187 1,786
Amortization of actuarial losses (gains) 5,556 774 12,556
Net curtailment gain (522) 0 0
Net periodic benefit (credit) cost (13,027) (12,303) 4,487
Other Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 444 535 682
Interest cost 1,281 1,661 1,605
Expected return on plan assets 0 0 0
Amortization of prior service cost (credit) (3,057) (2,871) (2,684)
Amortization of actuarial losses (gains) (348) (328) (234)
Net curtailment gain 0 0 0
Net periodic benefit (credit) cost $ (1,680) $ (1,003) $ (631)