Recent Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Reclassifisction of cororate expense to other components of net periodic pension and OPEB cost	$ 99	$ 177	$ 198	$ 354
ASU 2016-01
Defined Benefit Plan Disclosure [Line Items]
Reclassification of accumulated other comprehensive income related to marketable securities into beginning retained earnings			$ 46,100
ASU 2017-07
Defined Benefit Plan Disclosure [Line Items]
Reclassifisction of cororate expense to other components of net periodic pension and OPEB cost		$ 200		$ 400