Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 121,308	$ 122,902	$ 125,761	$ 139,024
Weighted average common shares outstanding - basic (in shares)	111,368	110,986	111,336	110,958
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units (in shares)	636	474	594	468
Weighted average common shares outstanding — diluted (in shares)	112,004	111,460	111,930	111,426
Net income attributable to common shareholders - basic (in dollars per share)	$ 1.09	$ 1.11	$ 1.13	$ 1.25
Net income attributable to common shareholders - diluted (in dollars per share)	$ 1.08	$ 1.10	$ 1.12	$ 1.25