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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic:
Net income (in usd)	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 184,443	$ 18,343	$ 169,459
Weighted average number of common shares during the period (in thousands)	41,814,000,000	41,785,000,000	41,762,000,000	41,610,000,000	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	41,737,876	41,570,492	41,364,546
Net income per share - basic (in usd per share)	$ 0.79	$ 0.61	$ 2.25	$ 0.77	$ (0.49)	$ 0.30	$ 0.14	$ 0.49	$ 3.03	$ 0.64	$ 0.05	$ 0.37	$ 4.42	$ 0.44	$ 4.10
Diluted:
Net income (in usd)	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 184,443	$ 18,343	$ 169,459
Weighted average number of common shares during the period (in thousands)	41,814,000,000	41,785,000,000	41,762,000,000	41,610,000,000	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	41,737,876	41,570,492	41,364,546
Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities (in thousands):
Stock options													79,000	100,000	112,000
CSUs related to deferred compensation for employees													0	25,000	25,000
RSUs related to incentive compensation													132,000	199,000	63,000
Total common and common equivalent shares adjusted to calculate diluted earnings per share (in thousands)	42,093,000,000	42,030,000,000	41,921,000,000	41,785,000,000	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	41,948,531	41,894,232	41,564,979
Net income per share - diluted (in usd per share)	$ 0.78	$ 0.60	$ 2.24	$ 0.77	$ (0.49)	$ 0.30	$ 0.14	$ 0.48	$ 3.00	$ 0.64	$ 0.05	$ 0.37	$ 4.40	$ 0.44	$ 4.08