Common Shares (Narrative) (Details) - USD ($)			3 Months Ended	9 Months Ended
	Aug. 02, 2021	Jun. 04, 2020	Sep. 30, 2021	Sep. 30, 2021	Oct. 29, 2021	Dec. 31, 2020
Number of common and preferred stock authorized to issue			185,000,000	185,000,000
Common shares, authorized			175,000,000	175,000,000		175,000,000
Preferred shares, authorized			10,000,000	10,000,000
Common Stock, Shares, Outstanding			102,200,000	102,200,000
Preferred Stock, Shares Outstanding			0	0
Unsecured Credit Facility [Member]
Maximum borrowing capacity under unsecured credit facility			$ 900,000,000	$ 900,000,000
2020 ATM program [Member]
Maximum aggregate offering price of common shares		$ 362,700,000
Stock Issued During Period, Value, New Issues				$ 358,800,000
Stock Issued During Period, Shares, New Issues				2,900,000
AveragePricePerCommonShareSold				$ 126.64
2020 ATM program [Member] | Subsequent Event [Member]
Maximum aggregate offering price of remaining common shares available for sale					$ 200,000
2021 ATM program
Maximum aggregate offering price of common shares	$ 500,000,000		1,500,000
Stock Issued During Period, Value, New Issues			$ 220,700,000
AveragePricePerCommonShareSold			$ 150.28
2021 ATM program | Subsequent Event [Member]
Maximum aggregate offering price of remaining common shares available for sale					278,200,000
April 2007 Repurchase Plan [Member]
Treasury stock allowed for repurchase			$ 500,000,000	$ 500,000,000
April 2007 Repurchase Plan [Member] | Subsequent Event [Member]
Share Repurchase Program, Remaining Authorized Repurchase Amount					$ 269,500,000