Basis of Presentation and 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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic:
Net income	$ 47,791	$ 36,474	$ 30,578	$ 18,472	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 133,315	$ 184,443	$ 18,343
Weighted average number of common shares during the period (in shares)	41,962,000	41,916,000	41,879,000	41,827,000	41,814,000	41,785,000	41,762,000	41,610,000	41,596,000	41,683,000	41,600,000	41,497,000	41,881,289	41,737,876	41,570,492
Net income per share - basic (in usd per share)	$ 1.14	$ 0.87	$ 0.73	$ 0.44	$ 0.79	$ 0.61	$ 2.25	$ 0.77	$ (0.49)	$ 0.30	$ 0.14	$ 0.49	$ 3.18	$ 4.42	$ 0.44
Diluted:
Net income	$ 47,791	$ 36,474	$ 30,578	$ 18,472	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 133,315	$ 184,443	$ 18,343
Weighted average number of common shares during the period (in shares)	41,962,000	41,916,000	41,879,000	41,827,000	41,814,000	41,785,000	41,762,000	41,610,000	41,596,000	41,683,000	41,600,000	41,497,000	41,881,289	41,737,876	41,570,492
Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities
Stock options (in shares)													44,000	79,000	100,000
CSUs related to deferred compensation for employees (in shares)													0	0	25,000
RSUs related to incentive compensation (in shares)													116,000	132,000	199,000
Total common and common equivalent shares adjusted to calculate diluted earnings per share (in shares)	42,156,000	42,058,000	41,996,000	42,019,000	42,093,000	42,030,000	41,921,000	41,785,000	41,911,000	41,850,000	41,735,000	41,653,000	42,040,892	41,948,531	41,894,232
Net income per share - diluted (in usd per share)	$ 1.13	$ 0.87	$ 0.73	$ 0.44	$ 0.78	$ 0.60	$ 2.24	$ 0.77	$ (0.49)	$ 0.30	$ 0.14	$ 0.48	$ 3.17	$ 4.40	$ 0.44