Basis of Presentation and Significant Accounting Policies (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Accounting Policies [Abstract]
Effective income tax rate, continuing operations		14.00%	12.90%
Tax benefit from foreign jurisdiction		$ 33,000
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 252,689	779,926			$ 252,689	$ 522,465
Corporate income tax rate					35.00%
Tax Cuts and Jobs Act of 2017, incomplete accounting, change in tax rate, deferred tax asset, provisional income tax expense	$ 6,500
ASU 2014-09 [Member] | Difference between revenue guidance in effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ (290,212)				269,776
ASU 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings						$ 2,100
Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Corporate income tax rate				21.00%