Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income	$ 20,359	$ 21,513
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities	(88)	(97)
Net income available to common stockholders	20,271	21,416
Effect of dilutive securities:
Participating securities		97
Net income for diluted net income per share	$ 20,271	$ 21,513
Reconciliation of shares
Shares for basic net income per share	39,451,000	39,366,000
Effect of dilutive securities: (Shares)
Total effect of dilutive securities (in shares)	3,000	246,000
Shares for diluted net income per share	39,454,000	39,612,000
Basic (in dollars per share)	$ 0.51	$ 0.54
Diluted (in dollars per share)	$ 0.51	$ 0.54
Stock options
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)	3,000	11,000
Performance-based stock units
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)		75,000
Number of shares granted	0
Participating Securities
Effect of dilutive securities: (Shares)
Participating securities		160,000