Share-Based Compensation - Summary of Share-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary of share-based compensation expense
Share-based compensation before income tax benefit	$ 2,294	$ 4,737	$ 5,056	$ 9,484
Income tax benefit	(580)	(1,752)	(1,276)	(3,524)
Share-based compensation, net of income tax benefit	1,714	2,985	3,780	5,960
Performance Unit Incentive Plan (“PUP”)
Summary of share-based compensation expense
Share-based compensation before income tax benefit	1,607	3,941	3,125	7,184
Restricted stock
Summary of share-based compensation expense
Share-based compensation before income tax benefit	609	672	1,779	2,069
Restricted stock units
Summary of share-based compensation expense
Share-based compensation before income tax benefit	$ 78	$ 124	$ 152	$ 231