Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,411	$ 3,532	$ 10,842	$ 9,967
Interest cost	6,348	6,482	18,953	19,114
Expected return on plan assets	(17,137)	(15,832)	(51,382)	(47,258)
Amortization of prior service cost (credit)	34	75	103	235
Amortization of actuarial losses (gains)	1,877	1,501	5,450	5,208
Net periodic benefit credit	(5,467)	(4,242)	(16,034)	(12,734)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	15	9	78	79
Interest cost	158	168	483	520
Amortization of prior service cost (credit)	561	(857)	(738)	(2,564)
Amortization of actuarial losses (gains)	(167)	(172)	(405)	(447)
Net periodic benefit credit	$ 567	$ (852)	$ (582)	$ (2,412)