Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	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 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