Postretirement Benefits - Reconciliation of Benefit Obligation and Plan Assets, Pension Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in plan assets:
Fair value at beginning of year	$ 322,898
Contributions	15,500	$ 14,600	$ 10,300
Fair value at end of year	307,596	322,898
Other Benefits [Member]
Changes in benefit obligation:
Obligation at beginning of year	22,895	20,860
Service cost—benefits earned during the period	0	0	354
Interest cost	1,075	646	1,030
Actuarial loss (gain)	(1,133)	1,700
Benefits paid	(358)	(311)
Obligation at end of year	22,479	22,895	20,860
Changes in plan assets:
Fair value at beginning of year	0	0
Return on assets	0	0
Contributions	358	311
Benefits paid	(358)	(311)
Fair value at end of year	0	0	0
Funded status at year end	(22,479)	(22,895)
Pension Plan [Member]
Changes in benefit obligation:
Obligation at beginning of year	477,426	383,859
Service cost—benefits earned during the period	15,902	12,925	14,984
Interest cost	19,887	19,270	17,043
Plan amendments	2,104	0
Actuarial loss (gain)	(19,226)	78,487
Benefits paid	(19,512)	(17,115)
Obligation at end of year	476,581	477,426	383,859
Changes in plan assets:
Fair value at beginning of year	322,898	291,753
Return on assets	(11,333)	33,641
Contributions	15,543	14,619
Benefits paid	(19,512)	(17,115)
Fair value at end of year	307,596	322,898	$ 291,753
Funded status at year end	$ (168,985)	$ (154,528)