Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Federal income tax rate		21.00%		35.00%
Reclassification of net deferred tax benefits from AOCI to retained earnings				$ 22,298
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
New accounting pronouncement, effect of change on operating income	$ (300)		$ (1,300)
Accounting Standards Update 2017-07 [Member] | Costs of Products Sold [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
New accounting pronouncement, effect of change on operating income	700		2,000
Accounting Standards Update 2017-07 [Member] | Selling, General and Administrative Expenses [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
New accounting pronouncement, effect of change on operating income	$ (400)		$ (700)
Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Federal income tax rate		21.00%		35.00%
Reclassification of net deferred tax benefits from AOCI to retained earnings		$ 22,300