Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic EPS [Abstract]
Weighted average common shares outstanding (in shares)	43,699,000	43,579,000	43,681,000	43,546,000
Net income available to common stockholders	$ 28,121	$ 21,359	$ 54,107	$ 41,638
Basic EPS (in dollars per share)	$ 0.64	$ 0.49	$ 1.24	$ 0.96
Diluted EPS [Abstract]
Weighted average common shares outstanding (in shares)	43,699,000	43,579,000	43,681,000	43,546,000
Dilutive effect of common stock options and restricted stock (in shares)	318,000	322,000	311,000	340,000
Weighted average common shares and common share equivalents (in shares)	44,017,000	43,901,000	43,992,000	43,886,000
Net income available to common stockholders	$ 28,121	$ 21,359	$ 54,107	$ 41,638
Diluted EPS (in dollars per share)	$ 0.64	$ 0.49	$ 1.23	$ 0.95
Stock Options [Member]
Earnings Per Share [Abstract]
Stock options excluded from calculation of diluted EPS (in shares)	1,500	3,250	1,500	1,144