Postretirement Benefits - Analysis of Impact on Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Impact on Other Comprehensive Income
Balance at January 1	$ (150,071)	$ (193,380)	$ (173,883)
Prior service cost	631	535	476
Net actuarial (gain) loss	7,843	14,560	11,960
Total amortization	8,474	15,095	12,436
Plan amendments	0	(2,377)	0
Experience gain (loss)	(40,636)	30,591	(31,933)
Balance at December 31	(182,233)	(150,071)	(193,380)
Other Benefits
Defined Benefit Plan, Impact on Other Comprehensive Income
Total amortization	$ 265	$ 92	$ 155