Accounting for Stock-based Compensation - Summary of Performance Shares Activity (Details) - Performance Shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number ofShares
Number of Shares, Non-vested Beginning Balance	129,974	58,822
Number of Shares, Granted	60,929	74,574	58,822
Number of Shares, Cancelled	(3,881)	(3,422)
Number of Shares, Non-vested Ending Balance	187,022	129,974	58,822
Number of Shares, expected to vest in the future	126,846
Weighted-Average Grant Date Fair Value
Weighted-Average Grant Date Fair Value, Non-vested Beginning Balance	$ 40.57	$ 44.21
Weighted-Average Grant Date Fair Value, Granted	38.81	37.75	$ 44.21
Weighted-Average Grant Date Fair Value, Cancelled	42.83	41.61
Weighted-Average Grant Date Fair Value, Non-vested Ending Balance	39.95	$ 40.57	$ 44.21
Weighted-Average Grant Date Fair Value, expected to vest in the future	$ 38.09
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Non-vested	$ 9,819
Aggregate Intrinsic Value, expected to vest in the future	$ 6,659