Stock-Based Compensation - Stock Appreciation Rights (Details) - Stock Appreciation Rights - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of SAR’s
Awards outstanding, beginning balance (in shares)	2,508,015	2,396,126	1,809,302
Granted (in shares)	0	176,824	655,855
Exercised (in shares)	(700,765)	(8,003)	(14,470)
Forfeited (in shares)	(105,981)	(56,932)	(54,561)
Awards outstanding, ending balance (in shares)	1,701,269	2,508,015	2,396,126
Number of SAR's - SAR's exercisable (in shares)	1,247,161
Number of SAR's - SAR's expected to vest (in shares)	863,626
Weighted Average Exercise Price (in dollars per share)
SAR's outstanding - beginning balance (in dollars per share)	$ 10.22	$ 10.33	$ 10.63
Granted (in dollars per share)		8.80	9.47
Exercised (in dollars per share)	10.36	8.57	9.21
Forfeited (in dollars per share)	9.66	10.75	10.30
SAR's outstanding - ending balance (in dollars per share)	10.20	$ 10.22	$ 10.33
Weighted Average Exercise Price - SAR's exercisable (in dollars per share)	10.34
Weighted Average Exercise Price - SAR's expected to vest (in dollars per share)	$ 10.40
Weighted Average Contractual Term - SAR's outstanding	5 years 8 months 12 days	6 years 8 months 12 days	7 years 7 months 6 days
Weighted Average Contractual Term - SAR's expected to vest	5 years 8 months 12 days