Postretirement Benefit Plans - Summary of Post-Retirement Benefit Costs by Component (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,894	$ 3,990
Interest cost	5,432	5,003
Expected return on assets	(5,782)	(5,323)
Amortization:
Prior service cost	120	81
Actuarial (gain) loss	2,424	3,534
Direct recognition of expense	0	0
Net periodic benefit cost	6,088	7,285
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	212	203
Expected return on assets	0	0
Amortization:
Prior service cost	0	0
Actuarial (gain) loss	8	23
Direct recognition of expense	34	176
Net periodic benefit cost	$ 254	$ 402