Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended							9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income	$ 34,937	$ 68,936	$ 20,359	$ 19,834	$ 20,616	$ 25,377	$ 21,513	$ 124,232	$ 67,506
Less net income allocated to non-controlling interests	(17)							(17)
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities	(89)				(80)			(268)	(269)
Income allocated to participating securities	(49)							(236)	(12)
Total net income allocated to participating securities	(138)				(80)			(504)	(281)
Net income available to common stockholders	34,782				20,536			123,711	67,225
Effect of dilutive securities:
Participating securities	138				80			504	281
Net income for diluted net income per share	$ 34,920				$ 20,616			$ 124,215	$ 67,506
Reconciliation of shares
Shares for basic net income per share	39,487				39,428			39,470	39,403
Effect of dilutive securities: (Shares)
Total effect of dilutive securities (in shares)	378				320			375	335
Shares for diluted net income per share	39,865				39,748			39,845	39,738
Basic (in dollars per share)	$ 0.88				$ 0.52			$ 3.13	$ 1.71
Diluted (in dollars per share)	$ 0.88				$ 0.52			$ 3.12	$ 1.70
Stock options
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)	4				9			3	11
Performance-based stock units
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)	217				170			217	170
Participating Securities
Effect of dilutive securities: (Shares)
Participating securities	157				141			155	154