Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Net income	$ 20,427	$ 30,844	$ 34,937	$ 68,936	$ 20,359
Less income allocated to non-controlling interests	(81)
Less income allocated to participating securities:
Non-forfeitable dividends on participating securities	(92)				(88)
Net income available to common stockholders	20,254				20,271
Effect of dilutive securities:
Participating securities	92
Net income for diluted net income per share	$ 20,346				$ 20,271
Reconciliation of shares
Shares for basic net income per share	39,532				39,451
Effect of dilutive securities: (Shares)
Total effect of dilutive securities (in shares)	342				3
Shares for diluted net income per share	39,874				39,454
Basic (in dollars per share)	$ 0.51				$ 0.51
Diluted (in dollars per share)	$ 0.51				$ 0.51
Stock options
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)	4				3
Performance-based stock units
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)	181				0
Participating Securities
Effect of dilutive securities: (Shares)
Participating securities	157