Earnings Per Share and Capital Structure Computation of Basic and Diluted Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Net income available to common shareholders	$ 31,615	$ 34,436	$ 56,968	$ 61,556
Weighted average common shares outstanding-Basic	39,887	40,670	39,900	40,608
Dilutive effect of convertible notes	0	0	0	0
Dilutive effect of stock-based awards	239	407	270	451
Weighted average common shares outstanding-Diluted	40,126	41,077	40,170	41,059
Earnings per common share-Basic	$ 0.79	$ 0.85	$ 1.43	$ 1.52
Earnings per common share-Diluted	$ 0.79	$ 0.84	$ 1.42	$ 1.50