Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share
Net income attributable to common shareholders	$ 132,458	$ 131,207	$ 148,224	$ 155,651
Average common shares outstanding - basic	110,565	109,962	110,546	109,898
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units	437	970	379	945
Average common shares outstanding - diluted	111,002	110,932	110,925	110,843
Earnings per average common share attributable to common shareholders -basic (in dollars per share)	$ 1.20	$ 1.19	$ 1.34	$ 1.42
Earnings per average common share attributable to common shareholders -diluted (in dollars per share)	$ 1.19	$ 1.18	$ 1.34	$ 1.40