Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 315,012	$ 276,072	$ 484,971	$ 466,827
Weighted average common shares outstanding - basic (in shares)	112,148	111,835	112,094	111,787
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units (in shares)	385	566	405	527
Weighted average common shares outstanding — diluted (in shares)	112,533	112,401	112,499	112,314
Earnings per weighted-average common share outstanding
Net income attributable to common shareholders - basic (in dollars per share)	$ 2.81	$ 2.47	$ 4.33	$ 4.18
Net income attributable to common shareholders - diluted (in dollars per share)	$ 2.80	$ 2.46	$ 4.31	$ 4.16