Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings per share — Basic:
Earnings from continuing operations	$ 11,700	$ 83,967	$ 75,694	$ 49,098	$ 65,944	$ 73,875	$ 62,575	$ 40,802	$ 220,459	$ 243,196	$ 200,899
Less: Distributed and undistributed earnings allocated to nonvested stock									(858)	(2,173)	(2,566)
Earnings from continuing operations available to common shareholders — Basic									219,601	241,023	198,333
Weighted average common shares outstanding— Basic									52,536	51,617	50,449
Earnings from continuing operations per common share — Basic	$ 0.22	$ 1.60	$ 1.43	$ 0.93	$ 1.25	$ 1.41	$ 1.21	$ 0.79	$ 4.18	$ 4.67	$ 3.93
Earnings per share — Diluted:
Earnings from continuing operations	11,700	83,967	75,694	49,098	65,944	73,875	62,575	40,802	220,459	243,196	200,899
Less: Distributed and undistributed earnings allocated to nonvested stock									(853)	(2,159)	(2,556)
Earnings from continuing operations available to common shareholders — Diluted									$ 219,606	$ 241,037	$ 198,343
Weighted average common shares outstanding— Basic									52,536	51,617	50,449
Effect of dilutive equity awards									500	454	291
Weighted average common shares outstanding— Diluted									53,036	52,071	50,740
Earnings from continuing operations per common share — Diluted	$ 0.22	$ 1.58	$ 1.42	$ 0.92	$ 1.24	$ 1.40	$ 1.19	$ 0.79	$ 4.14	$ 4.63	$ 3.91
Anti-dilutive equity awards and market-based restrictive stock rights not included above									161	785	2,278