Postretirement Benefits - Reconciliation of Benefit Obligation and Plan Assets, Pension Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in plan assets:
Contributions	$ 21,300	$ 15,800	$ 15,500
Pension Benefits
Changes in benefit obligation:
Obligation at beginning of year	527,522	476,581
Service cost	17,942	15,502	15,902
Interest cost	22,124	21,631	19,887
Plan amendments	0	0
Actuarial loss (gain)	55,369	34,667
Benefits paid	(20,351)	(20,859)
Obligation at end of year	602,606	527,522	476,581
Changes in plan assets:
Fair value at beginning of year	328,871	307,596
Return on assets	47,832	26,377
Contributions	21,272	15,757
Benefits paid	(20,351)	(20,859)
Fair value at end of year	377,624	328,871	307,596
Funded status at year end	(224,982)	(198,651)
Other Benefits
Changes in benefit obligation:
Obligation at beginning of year	23,721	22,479
Service cost	0	0	0
Interest cost	1,132	1,139	1,075
Actuarial loss (gain)	1,045	412
Benefits paid	(285)	(309)
Obligation at end of year	25,613	23,721	22,479
Changes in plan assets:
Fair value at beginning of year	0	0
Return on assets	0	0
Contributions	285	309
Benefits paid	(285)	(309)
Fair value at end of year	0	0	$ 0
Funded status at year end	$ (25,613)	$ (23,721)