Postretirement Benefits Other Than Pensions - Funded Status of Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 282,183	$ 278,915
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	32,132	39,126
Service cost	640	821
Interest cost	1,085	1,455
Employee contributions	338	389
Actuarial (gain) loss	(1,916)	(8,048)
Benefits paid	(2,157)	(1,611)
Benefit obligation at end of year	30,122	32,132
Fair value of plan assets at end of year	0	0
Funded status	$ (30,122)	$ (32,132)