Postretirement Benefits - Components of Net Periodic Pension Costs and Post-Retirement Benefit Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net amortization	$ 10,168	$ 14,586	$ 10,285
Net periodic pension cost	24,202	29,230	23,463
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	15,502	15,902	12,925
Interest cost on projected benefit obligation	21,631	19,887	19,270
Expected return on assets	(23,127)	(21,204)	(19,031)
Net amortization	10,135	14,465	10,283
Recognition of actuarial loss	61	180	16
Net periodic pension cost	24,202	29,230	23,463
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	0	0	0
Interest cost on projected benefit obligation	1,139	1,075	646
Expected return on assets	0	0	0
Net amortization	33	120	2
Recognition of actuarial loss	(132)	367	(256)
Net periodic pension cost	$ 1,040	$ 1,562	$ 392