Summary of Significant Accounting Policies - Calculation of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic:
Net income (in usd)	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 18,343	$ 169,459	$ 83,765
Weighted average number of common shares during the period (in thousands)	41,596,000,000	41,683,000,000	41,600,000,000	41,497,000,000	41,419,000,000	41,433,000,000	41,368,000,000	41,135,000,000	41,093,000,000	41,092,000,000	41,082,000,000	41,297,000,000	41,570,492	41,364,546	41,158,349
Net income per share - basic (in usd per share)	$ (0.49)	$ 0.30	$ 0.14	$ 0.49	$ 3.03	$ 0.64	$ 0.05	$ 0.37	$ 0.48	$ 0.66	$ 0.29	$ 0.61	$ 0.44	$ 4.10	$ 2.04
Diluted:
Net income (in usd)	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 18,343	$ 169,459	$ 83,765
Weighted average number of common shares during the period (in thousands)	41,596,000,000	41,683,000,000	41,600,000,000	41,497,000,000	41,419,000,000	41,433,000,000	41,368,000,000	41,135,000,000	41,093,000,000	41,092,000,000	41,082,000,000	41,297,000,000	41,570,492	41,364,546	41,158,349
Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities (in thousands):
Stock options													100,000	112,000	100,000
CSUs related to deferred compensation for employees													25,000	25,000	52,000
RSUs related to incentive compensation													199,000	63,000	166,000
Total common and common equivalent shares adjusted to calculate diluted earnings per share (in thousands)	41,911,000,000	41,850,000,000	41,735,000,000	41,653,000,000	41,718,000,000	41,575,000,000	41,493,000,000	41,342,000,000	41,482,000,000	41,347,000,000	41,314,000,000	41,492,000,000	41,894,232	41,564,979	41,475,516
Net income per share - diluted (in usd per share)	$ (0.49)	$ 0.30	$ 0.14	$ 0.48	$ 3.00	$ 0.64	$ 0.05	$ 0.37	$ 0.48	$ 0.65	$ 0.29	$ 0.61	$ 0.44	$ 4.08	$ 2.02