Borrowings - Credit Agreement (Details) - USD ($) $ in Thousands	Jan. 26, 2018	Dec. 31, 2017	Dec. 31, 2016
Term Debt
Term loans		$ 3,104	$ 3,098
Obligations under capital lease			3,269
Total		3,104	6,367
Revolving Credit
Credit Agreement
Current borrowing capacity	$ 150,000
Credit facility initial term	364 days
Unused commitment fee (as a percent)	0.75%
Incremental interest rate payable in event of default (as a percent)	2.00%
Amount outstanding under Credit Agreement		$ 0	$ 0
Revolving Credit | Subsequent event
Credit Agreement
Current borrowing capacity	$ 150,000
Credit facility initial term	364 days
Revolving Credit | Federal funds rate
Credit Agreement
Interest, margin over base rate (as a percent)	2.50%