Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 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		$ (1,100)
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		1,300
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		$ (200)
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