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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 47,595	$ 47,565	$ 121,334	$ 107,550
Income allocated to participating securities	(334)	(342)	(861)	(749)
Net income available to common shareholders	$ 47,261	$ 47,223	$ 120,473	$ 106,801
Denominator:
Weighted average common shares outstanding – basic (in shares)	56,161	55,630	56,059	55,904
Basic earnings per share (in dollars per share)	$ 0.84	$ 0.85	$ 2.15	$ 1.91
Numerator:
Income allocated to participating securities	$ (333)	$ (341)	$ (857)	$ (746)
Net income available to common shareholders	$ 47,262	$ 47,224	$ 120,477	$ 106,804
Denominator:
Weighted average common shares outstanding – basic (in shares)	56,161	55,630	56,059	55,904
Diluted effect of stock options and PVRSUs (in shares)	345	281	358	296
Weighted average common shares outstanding – diluted (in shares)	56,506	55,911	56,417	56,200
Diluted earnings per share (in dollars per share)	$ 0.84	$ 0.84	$ 2.14	$ 1.90