Common Shares (Narrative) (Details) - USD ($)	1 Months Ended			9 Months Ended
	Oct. 27, 2017	May 31, 2017	Nov. 30, 2014	Sep. 30, 2017	Sep. 30, 2016	May 05, 2017	Dec. 31, 2016
Proceeds from issuance of common shares				$ 445,038,000	$ 0
Number of common and preferred stock authorized to issue				185,000,000
Common shares, authorized				175,000,000			175,000,000
Preferred shares, authorized				10,000,000
Common Stock, Shares, Outstanding				92,700,000
Preferred Stock, Shares Outstanding				0
2014 ATM program [Member]
Maximum aggregate offering price of remaining common shares available for sale						$ 315,300,000
Maximum aggregate offering price of common shares			$ 331,300,000
2017 ATM program [Member]
Total net consideration				$ 2,513,600
Common shares issued				28,100
Average price per share				$ 90.44
Maximum aggregate offering price of common shares		$ 315,300,000
April 2007 Repurchase Plan [Member]
Treasury stock allowed for repurchase				$ 500,000,000
Unsecured Credit Facility [Member]
Line of Credit Facility, Current Borrowing Capacity				$ 600,000,000
Subsequent Event [Member] | 2017 ATM program [Member]
Common shares issued	0
Maximum aggregate offering price of remaining common shares available for sale	$ 312,800,000
Subsequent Event [Member] | April 2007 Repurchase Plan [Member]
Share Repurchase Program, Remaining Authorized Repurchase Amount	$ 269,800,000
2017 equity offering [Member]
Common shares issued				4,800,000
Proceeds from issuance of common shares				$ 442,500,000