Postretirement Benefits - Components of Net Periodic Pension Costs and Post-Retirement Benefit Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net amortization	$ 12,436	$ 10,168	$ 14,586
Net periodic pension cost	28,828	24,202	29,230
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	17,942	15,502	15,902
Interest cost on projected benefit obligation	22,124	21,631	19,887
Expected return on assets	(23,597)	(23,127)	(21,204)
Net amortization	12,281	10,135	14,465
Recognition of actuarial loss	78	61	180
Net periodic pension cost	28,828	24,202	29,230
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	0	0	0
Interest cost on projected benefit obligation	1,132	1,139	1,075
Expected return on assets	0	0	0
Net amortization	155	33	120
Recognition of actuarial loss	167	(132)	367
Net periodic pension cost	$ 1,454	$ 1,040	$ 1,562