Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,548	$ 5,776	$ 13,402	$ 11,082
Interest cost	8,673	9,331	18,030	17,938
Expected return on assets	(10,071)	(9,822)	(20,613)	(18,848)
Prior service cost (credit)	113	91	155	175
Actuarial loss (gain)	3,551	3,146	7,087	6,036
Settlement charge				59
Special termination benefit		638		764
Net periodic benefit cost (credit)	8,814	9,160	18,061	17,206
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	24	40	43
Interest cost	198	259	365	432
Prior service cost (credit)	(471)	(609)	(871)	(981)
Actuarial loss (gain)	(99)	(139)	(182)	(249)
Special termination benefit		(8)		(8)
Net periodic benefit cost (credit)	$ (350)	$ (473)	$ (648)	$ (763)