Postretirement Benefit Plans - Schedule of Post-Retirement Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,991	$ 3,231	$ 7,981	$ 6,463
Interest cost	5,004	4,819	10,006	9,638
Expected return on assets	(5,323)	(4,752)	(10,646)	(9,505)
Amortization:
Prior service cost	82	529	163	1,057
Actuarial (gain) loss	3,534	2,047	7,068	4,092
Direct recognition of expense	0	0	0	0
Net periodic benefit cost	7,288	5,874	14,572	11,745
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	(7)	0	49
Interest cost	204	150	407	297
Expected return on assets	0	0	0	0
Amortization:
Prior service cost	0	0	0	0
Actuarial (gain) loss	37	0	60	0
Direct recognition of expense	151	(42)	327	(104)
Net periodic benefit cost	$ 392	$ 101	$ 794	$ 242