Postretirement Benefit Plans - Defined Contribution Plans - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Supplemental plan liability balances	$ 1,661	$ 1,767
Defined contribution plan expense	12,000	11,132	$ 9,616
Statutory Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan expense	398	290	$ 1,375
Defined Contribution Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Balance of trust assets	$ 1,587	$ 1,692