Postretirement Benefit Plans - Defined Contribution Plans - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Contribution Plan Disclosure [Line Items]
Supplemental plan liability balances	$ 1,827	$ 1,776
Defined contribution plan expense	9,616	8,184	$ 9,304
Statutory Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan expense	1,375	145	$ 1,999
Defined Contribution Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Balance of trust assets	$ 1,762	$ 1,741