Earnings Per Share Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings per share - Basic:
Earnings from continuing operations	$ 65,944	$ 73,875	$ 62,575	$ 40,802	$ 54,945	$ 64,311	$ 46,767	$ 34,876	$ 243,196	$ 200,899	$ 171,368
Less: Distributed and undistributed earnings allocated to nonvested stock									(2,173)	(2,566)	(2,751)
Earnings from continuing operations available to common shareholders - Basic									241,023	198,333	168,617
Weighted average common shares outstanding - Basic									51,617	50,449	50,500
Earnings from continuing operations per common share - Basic (USD per share)	$ 1.25	$ 1.41	$ 1.21	$ 0.79	$ 1.07	$ 1.26	$ 0.92	$ 0.68	$ 4.67	$ 3.93	$ 3.34
Earnings per share - Diluted:
Earnings from continuing operations	65,944	73,875	62,575	40,802	54,945	64,311	46,767	34,876	243,196	200,899	171,368
Less: Distributed and undistributed earnings allocated to nonvested stock									(2,159)	(2,556)	(2,737)
Earnings from continuing operations available to common shareholders - Diluted									$ 241,037	$ 198,343	$ 168,631
Weighted average common shares outstanding - Basic									51,617	50,449	50,500
Effect of dilutive options (shares)									454	291	378
Weighted average common shares outstanding - Diluted									52,071	50,740	50,878
Earnings from continuing operations per common share - Diluted (USD per share)	$ 1.24	$ 1.40	$ 1.19	$ 0.79	$ 1.07	$ 1.26	$ 0.91	$ 0.68	$ 4.63	$ 3.91	$ 3.31
Anti-dilutive options not included above (shares)									785	2,278	1,514