Postretirement Benefits - Analysis of Impact on Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Impact on Other Comprehensive Income
Balance at January 1	$ (173,883)	$ (152,149)	$ (152,999)
Amortization of prior service cost	476	477	377
Amortization of net actuarial (gain) loss	11,960	9,691	14,209
Total amortization	12,436	10,168	14,586
Plan amendments	0	0	(2,104)
Experience gain (loss)	(31,933)	(31,902)	(11,632)
Balance at December 31	(193,380)	(173,883)	(152,149)
Other Benefits
Defined Benefit Plan, Impact on Other Comprehensive Income
Balance at January 1	1,959
Total amortization	155	33	$ 120
Balance at December 31	$ 2,682	$ 1,959