Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,710	$ 3,267	$ 7,431	$ 6,435
Interest cost	6,296	6,362	12,605	12,632
Expected return on plan assets	(17,122)	(15,713)	(34,245)	(31,426)
Amortization of prior service cost (credit)	34	80	69	160
Amortization of actuarial losses (gains)	1,787	1,854	3,573	3,707
Net periodic benefit credit	(5,295)	(4,150)	(10,567)	(8,492)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	32	34	63	70
Interest cost	162	176	325	352
Amortization of prior service cost (credit)	(650)	(854)	(1,299)	(1,707)
Amortization of actuarial losses (gains)	(119)	(136)	(238)	(275)
Net periodic benefit credit	$ (575)	$ (780)	$ (1,149)	$ (1,560)