Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,457	$ 5,542	$ 20,060	$ 16,624
Interest cost	9,729	8,970	27,074	26,908
Expected return on assets	(10,691)	(9,425)	(29,818)	(28,272)
Prior service cost (credit)	34	87	237	262
Actuarial loss (gain)	3,604	3,018	10,643	9,055
Settlement charge				59
Special termination benefit				764
Net periodic benefit cost (credit)	10,133	8,192	28,196	25,400
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	20	22	60	65
Interest cost	182	216	547	648
Prior service cost (credit)	(436)	(491)	(1,307)	(1,471)
Actuarial loss (gain)	(91)	(125)	(273)	(374)
Special termination benefit				(8)
Net periodic benefit cost (credit)	$ (325)	$ (378)	$ (973)	$ (1,140)