Basis of Presentation - Recently Adopted Accounting Standards (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash paid on behalf of employees for shares withheld	$ 5,111	$ 5,155
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease commitment	$ 25,200