Weighted Average Shares Outstanding and Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Weighted Avg Shares Outstanding and Earnings Per Share [Line Items]
Net income allocated to common shareholders for basic calculation	$ 45,982	$ 37,191	$ 36,325	$ 40,949
Weighted average common shares outstanding, basic	39,844	39,599	39,774	39,539
Dilutive effect of common stock options and restricted stock units	543	714	593	666
Weighted average common shares outstanding, diluted	40,387	40,313	40,367	40,205
Basic (in dollars per share)	$ 1.15	$ 0.94	$ 0.91	$ 1.04
Diluted (in dollars per share)	$ 1.14	$ 0.92	$ 0.90	$ 1.02