Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 243,961	$ 226,163	$ 392,185	$ 381,814
Average common shares outstanding - basic	110,686	110,009	110,579	109,935
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units	417	1,044	383	978
Average common shares outstanding — diluted	111,103	111,053	110,962	110,913
Earnings per average common share attributable to common shareholders -basic (in dollars per share)	$ 2.20	$ 2.06	$ 3.55	$ 3.47
Earnings per average common share attributable to common shareholders -diluted (in dollars per share)	$ 2.20	$ 2.04	$ 3.53	$ 3.44