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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic:
Net income (in usd)	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 21,040	$ 21,984	$ 16,183	$ 34,275	$ 169,459	$ 83,765	$ 93,482
Weighted average number of common shares during the period (in thousands)	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	41,564,000,000	41,852,000,000	41,990,000,000	41,950,000,000	41,364,546	41,158,349	41,914,864
Net income per share - basic (in usd per share)	$ 3.03	$ 0.64	$ 0.05	$ 0.37	$ 0.48	$ 0.66	$ 0.29	$ 0.61	$ 0.51	$ 0.53	$ 0.39	$ 0.82	$ 4.10	$ 2.04	$ 2.23
Diluted:
Net income (in usd)	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 21,040	$ 21,984	$ 16,183	$ 34,275	$ 169,459	$ 83,765	$ 93,482
Weighted average number of common shares during the period (in thousands)	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	41,564,000,000	41,852,000,000	41,990,000,000	41,950,000,000	41,364,546	41,158,349	41,914,864
Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities (in thousands):
Stock options													112,000	100,000	158,000
CSUs related to deferred compensation for employees													25,000	52,000	55,000
RSUs related to incentive compensation													63,000	166,000	297,000
Total common and common equivalent shares adjusted to calculate diluted earnings per share (in thousands)	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	42,127,000,000	42,305,000,000	42,425,000,000	42,300,000,000	41,564,979	41,475,516	42,424,806
Net income per share - diluted (in usd per share)	$ 3.00	$ 0.64	$ 0.05	$ 0.37	$ 0.48	$ 0.65	$ 0.29	$ 0.61	$ 0.50	$ 0.52	$ 0.38	$ 0.81	$ 4.08	$ 2.02	$ 2.20