Postretirement Benefits - Analysis of Impact on Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Impact on Other Comprehensive Income
Balance at January 1	$ (152,149)	$ (152,999)	$ (97,467)
Amortization of prior service cost	477	377	2,113
Amortization of net actuarial (gain) loss	9,691	14,209	8,172
Total amortization	10,168	14,586	10,285
Plan amendments	0	(2,104)	0
Experience gain (loss)	(31,902)	(11,632)	(65,817)
Balance at December 31	(173,883)	(152,149)	(152,999)
Other Benefits
Defined Benefit Plan, Impact on Other Comprehensive Income
Balance at January 1	1,447
Total amortization	33	120	$ 2
Balance at December 31	$ 1,959	$ 1,447