Share-Based Compensation - Summary of Share-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of share-based compensation expense
Share-based compensation before income tax benefit	$ 10,969	$ 8,038	$ 3,848
Income tax benefit	(4,079)	(2,988)	(1,454)
Share-based compensation, net of income tax benefit	6,890	5,050	2,394
Performance unit incentive plan (“PUP”)
Summary of share-based compensation expense
Share-based compensation before income tax benefit	8,088	5,703	1,692
Restricted stock
Summary of share-based compensation expense
Share-based compensation before income tax benefit	2,594	2,073	2,111
Restricted stock units
Summary of share-based compensation expense
Share-based compensation before income tax benefit	$ 287	$ 262	$ 45