Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Income from continuing operations attributable to common shareholders	$ 397,595	$ 406,074	$ 387,380
Average common shares outstanding — basic (in shares)	110,626	109,984	109,510
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Contingently issuable performance shares and restricted stock units	552	822	1,017
Average common shares outstanding — diluted	111,178	110,806	110,527
Income from continuing operations attributable to common shareholders — basic (in dollars per share)	$ 3.59	$ 3.69	$ 3.54
Income from continuing operations attributable to common shareholders — diluted (in dollars per share)	$ 3.58	$ 3.66	$ 3.50