Pension Plans and Other Postretirement Benefits - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	$ 20,812	$ 14,183
Amendments and curtailments	(30,088)	(28,308)
Amortization of prior service (cost) credit	802	4,587	$ 7,364
Amortization of actuarial (loss) gain	(28,275)	(30,205)	$ (44,931)
Total recognized in other comprehensive income, pre-tax	(36,749)	(39,743)
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	29,323	24,736
Amendments and curtailments		(28,308)
Amortization of prior service (cost) credit	(557)	3,228
Amortization of actuarial (loss) gain	(29,263)	(30,145)
Total recognized in other comprehensive income, pre-tax	(497)	(30,489)
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	(8,511)	(10,553)
Amendments and curtailments	(30,088)
Amortization of prior service (cost) credit	1,359	1,359
Amortization of actuarial (loss) gain	988	(60)
Total recognized in other comprehensive income, pre-tax	$ (36,252)	$ (9,254)