Postretirement Benefits Other Than Pensions - Funded Status of Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 344,760	$ 282,183
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	30,122	32,132
Service cost	586	640
Interest cost	960	1,085
Employee contributions	256	338
Actuarial (gain) loss	1,622	(1,916)
Benefits paid	(1,680)	(2,157)
Benefit obligation at end of year	31,866	30,122
Fair value of plan assets at end of year	0	0
Funded status	$ (31,866)	$ (30,122)