Earnings Per Share Computation Of Basic And Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 79,959	$ 57,189	$ 184,884	$ 132,143
Income allocated to participating securities	(447)	(402)	(1,081)	(938)
Net income available to common shareholders	$ 79,512	$ 56,787	$ 183,803	$ 131,205
Denominator:
Weighted average common shares outstanding – basic (in shares)	56,025	56,161	56,283	56,059
Basic earnings per share (in dollars per share)	$ 1.42	$ 1.01	$ 3.27	$ 2.34
Numerator:
Income allocated to participating securities	$ (444)	$ (400)	$ (1,073)	$ (933)
Net income available to common shareholders	$ 79,515	$ 56,789	$ 183,811	$ 131,210
Denominator:
Weighted average common shares outstanding – basic (in shares)	56,025	56,161	56,283	56,059
Diluted effect of stock options and PVRSUs (in shares)	405	344	505	357
Weighted average common shares outstanding – diluted (in shares)	56,430	56,505	56,788	56,416
Diluted earnings per share (in dollars per share)	$ 1.41	$ 1.01	$ 3.24	$ 2.33