Postretirement Benefits - Reconciliation of Benefit Obligation and Plan Assets, Pension Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in plan assets:
Fair value at beginning of year	$ 307,596
Contributions	15,800	$ 15,500	$ 14,600
Fair value at end of year	328,871	307,596
Other Benefits
Changes in benefit obligation:
Obligation at beginning of year	22,479	22,895
Service cost—benefits earned during the period	0	0	0
Interest cost	1,139	1,075	646
Actuarial loss (gain)	412	(1,133)
Benefits paid	(309)	(358)
Obligation at end of year	23,721	22,479	22,895
Changes in plan assets:
Fair value at beginning of year	0	0
Return on assets	0	0
Contributions	309	358
Benefits paid	(309)	(358)
Fair value at end of year	0	0	0
Funded status at year end	(23,721)	(22,479)
Pension Benefits
Changes in benefit obligation:
Obligation at beginning of year	476,581	477,426
Service cost—benefits earned during the period	15,502	15,902	12,925
Interest cost	21,631	19,887	19,270
Plan amendments	0	2,104
Actuarial loss (gain)	34,667	(19,226)
Benefits paid	(20,859)	(19,512)
Obligation at end of year	527,522	476,581	477,426
Changes in plan assets:
Fair value at beginning of year	307,596	322,898
Return on assets	26,377	(11,333)
Contributions	15,757	15,543
Benefits paid	(20,859)	(19,512)
Fair value at end of year	328,871	307,596	$ 322,898
Funded status at year end	$ (198,651)	$ (168,985)