Share-Based Compensation - Summary of Share-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Summary of share-based compensation expense
Share-based compensation before income tax benefit	$ 2,045	$ 2,748	$ 2,762	$ 4,747
Income tax benefit	(515)	(1,028)	(696)	(1,772)
Share-based compensation, net of income tax benefit	1,530	1,720	2,066	2,975
Performance unit incentive plan (“PUP”)
Summary of share-based compensation expense
Share-based compensation before income tax benefit	1,324	1,927	1,518	3,243
Restricted stock
Summary of share-based compensation expense
Share-based compensation before income tax benefit	667	774	1,170	1,397
Restricted stock units
Summary of share-based compensation expense
Share-based compensation before income tax benefit	$ 54	$ 47	$ 74	$ 107