Earnings Per Share and Capital Structure Computation of Basic and Diluted Earnings per Share (Details) (USD $) In Thousands, except Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net income available to common shareholders	$ 34,436	$ 25,311	$ 61,556	$ 50,561
Weighted Average Number of Shares Outstanding, Basic	40,670,000	40,329,000	40,608,000	40,261,000
Dilutive effect of convertible notes	0	283,000	0	206,000
Dilutive effect of stock-based awards	407,000	549,000	451,000	544,000
Weighted Average Number of Shares Outstanding, Diluted	41,077,000	41,161,000	41,059,000	41,011,000
Earnings Per Share, Basic	$ 0.85	$ 0.63	$ 1.52	$ 1.26
Earnings Per Share, Diluted	$ 0.84	$ 0.61	$ 1.50	$ 1.23