Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 257,116	$ 243,961	$ 396,140	$ 392,185
Weighted average common shares outstanding - basic (in shares)	111,036	110,686	110,984	110,579
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units (in shares)	580	417	506	383
Weighted average common shares outstanding — diluted (in shares)	111,616	111,103	111,490	110,962
Earnings per average common share attributable to common shareholders - basic (in dollars per share)	$ 2.32	$ 2.20	$ 3.57	$ 3.55
Earnings per average common share attributable to common shareholders - diluted (in dollars per share)	$ 2.30	$ 2.20	$ 3.55	$ 3.53