Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,168	$ 3,313
Interest cost	6,270	6,434
Expected return on plan assets	(15,713)	(14,583)
Amortization of prior service cost (credit)	80	151
Amortization of actuarial losses (gains)	1,853	2,083
Net periodic benefit credit	(4,342)	(2,602)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	36	67
Interest cost	176	254
Amortization of prior service cost (credit)	(853)	(850)
Amortization of actuarial losses (gains)	(139)	(118)
Net periodic benefit credit	$ (780)	$ (647)