Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic EPS [Abstract]
Weighted average common shares outstanding (in shares)	43,714,000	43,595,000	43,692,000	43,563,000
Net income available to common stockholders	$ 29,807	$ 22,876	$ 83,914	$ 64,514
Basic EPS (in dollars per share)	$ 0.68	$ 0.52	$ 1.92	$ 1.48
Diluted EPS [Abstract]
Weighted average common shares outstanding (in shares)	43,714,000	43,595,000	43,692,000	43,563,000
Dilutive effect of common stock options and restricted stock (in shares)	337,000	320,000	317,000	329,000
Weighted average common shares and common share equivalents (in shares)	44,051,000	43,915,000	44,009,000	43,892,000
Net income available to common stockholders	$ 29,807	$ 22,876	$ 83,914	$ 64,514
Diluted EPS (in dollars per share)	$ 0.68	$ 0.52	$ 1.91	$ 1.47
Stock Options [Member]
Earnings Per Share [Abstract]
Stock options excluded from calculation of diluted EPS (in shares)	1,500	3,250	1,500	3,014