Significant Accounting Policies - Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense			$ 14,940	$ 20,072	$ 25,150
Scenario, Forecast | Accounting Standards Updated 2016-09 | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense	$ 600	$ 400
Scenario, Forecast | Accounting Standards Updated 2016-09 | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense	$ 900	$ 700