Earnings Per Share Computation Of Basic And Diluted Earnings Per Common Share (Details) (USD $) In Thousands, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net income	$ 42,302	$ 40,494	$ 85,610	$ 83,298
Weighted average common shares outstanding - basic	59,182	58,965	59,169	58,948
Basic earnings per share	$ 0.71	$ 0.68	$ 1.43	$ 1.40
Diluted effect of stock options and PVRSUs	36	66	44	80
Weighted average common shares outstanding - diluted	59,218	59,031	59,213	59,028
Diluted earnings per share	$ 0.71	$ 0.68	$ 1.43	$ 1.40
Earnings Per Share, Basic [Member]
Net income	42,302	40,494	85,610	83,298
Income allocated to participating securities	(417)	(426)	(848)	(865)
Net income available to common shareholders	41,885	40,068	84,762	82,433
Basic earnings per share	$ 0.71	$ 0.68	$ 1.43	$ 1.40
Earnings Per Share, Diluted [Member]
Net income	42,302	40,494	85,610	83,298
Income allocated to participating securities	(417)	(425)	(848)	(864)
Net income available to common shareholders	$ 41,885	$ 40,069	$ 84,762	$ 82,434
Diluted earnings per share	$ 0.71	$ 0.68	$ 1.43	$ 1.40