Postretirement Benefit Plans - Defined Contribution Plans - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Supplemental plan liability balances	$ 1,519	$ 1,661
Defined contribution plan expense	11,799	12,000	$ 11,132
Statutory Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan expense	374	398	$ 290
Defined Contribution Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Balance of trust assets	$ 1,444	$ 1,587