Employee Retirement Plans - Defined Benefit Plans - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	$ 0	$ 0	$ 0	$ 0
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated payments for the current year	2,100,000		2,100,000
OPEB Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Changes in OPEB benefit obligation			700,000
Amount recognized in other comprehensive income			700,000
Estimated payments for the current year	$ 7,300,000		$ 7,300,000