Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 122,902	$ 132,458	$ 139,024	$ 148,224
Weighted average common shares outstanding - basic	110,986	110,565	110,958	110,546
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units	474	437	468	379
Weighted average common shares outstanding — diluted	111,460	111,002	111,426	110,925
Earnings per average common share attributable to common shareholders -basic (in dollars per share)	$ 1.11	$ 1.20	$ 1.25	$ 1.34
Earnings per average common share attributable to common shareholders -diluted (in dollars per share)	$ 1.10	$ 1.19	$ 1.25	$ 1.34