Earnings Per Share Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings per share - Basic:
Earnings from continuing operations	$ 54,945	$ 64,311	$ 46,767	$ 34,876	$ 47,664	$ 56,933	$ 40,914	$ 25,857	$ 200,899	$ 171,368	$ 124,608
Less: Distributed and undistributed earnings allocated to nonvested stock									(2,566)	(2,751)	(1,759)
Earnings from continuing operations available to common shareholders - Basic									198,333	168,617	122,849
Weighted average common shares outstanding - Basic									50,449	50,500	51,717
Earnings from continuing operations per common share - Basic (USD per share)	$ 1.07	$ 1.26	$ 0.92	$ 0.68	$ 0.93	$ 1.11	$ 0.80	$ 0.50	$ 3.93	$ 3.34	$ 2.38
Earnings per share - Diluted:
Earnings from continuing operations	54,945	64,311	46,767	34,876	47,664	56,933	40,914	25,857	200,899	171,368	124,608
Less: Distributed and undistributed earnings allocated to nonvested stock									(2,556)	(2,737)	(1,756)
Earnings from continuing operations available to common shareholders - Diluted									$ 198,343	$ 168,631	$ 122,852
Weighted average common shares outstanding - Basic									50,449	50,500	51,717
Effect of dilutive options (shares)									291	378	167
Weighted average common shares outstanding - Diluted									50,740	50,878	51,884
Earnings from continuing operations per common share - Diluted (USD per share)	$ 1.07	$ 1.26	$ 0.91	$ 0.68	$ 0.92	$ 1.10	$ 0.79	$ 0.50	$ 3.91	$ 3.31	$ 2.37
Anti-dilutive options not included above (shares)									2,278	1,514	1,654