Basis of Presentation and Significant Accounting Policies (Detail) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Accounting Policies [Abstract]
Effective income tax rate, continuing operations	21.00%		24.20%
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 587,483	$ 252,689			$ 252,689	$ 522,465
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent					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	$ (275,540)					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]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				21.00%