Postretirement Benefits - Components of Net Periodic Pension Costs and Post-Retirement Benefit Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Net amortization	$ 14,586	$ 10,285	$ 18,366
Net periodic pension cost	29,230	23,463	33,122
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	15,902	12,925	14,984
Interest cost on projected benefit obligation	19,887	19,270	17,043
Expected return on assets	(21,204)	(19,031)	(17,429)
Net amortization	14,465	10,283	18,143
Recognition of actuarial loss	180	16	381
Net periodic pension cost	29,230	23,463	33,122
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	0	0	354
Interest cost on projected benefit obligation	1,075	646	1,030
Expected return on assets	0	0	0
Net amortization	120	2	224
Recognition of actuarial loss	367	(256)	0
Net periodic pension cost	$ 1,562	$ 392	$ 1,608