Notes Payable (Narrative) (Details)			3 Months Ended	12 Months Ended
		Jun. 06, 2019 USD ($)	Dec. 31, 2019 USD ($) yr	Dec. 31, 2019 USD ($) yr	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 01, 2019 USD ($)	Oct. 01, 2018 USD ($)	Sep. 30, 2018
Secured Debt			$ 0	$ 0	$ 485,176,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net			19,900,000	$ 19,900,000	13,900,000
Current Fiscal Year End Date				--12-31
Unsecured Debt			2,524,099,000	$ 2,524,099,000	1,836,427,000
Total notes payable			2,524,100,000	2,524,100,000	2,321,600,000
Loss on early retirement of debt			11,995,000	11,995,000	0	$ 323,000
Available amount under unsecured credit facility			$ 847,100,000	847,100,000
Payment for Debt Extinguishment or Debt Prepayment Cost				$ 12,000,000
Weighted average interest rate on floating rate debt	[1]		3.80%	3.80%
Weighted average maturity of indebtedness (including unsecured credit facility and unsecured short-term borrowings) (in years) | yr			8.9	8.9
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility			$ 50,000,000	$ 50,000,000
Outstanding balance under credit facility			8,900,000	8,900,000
Commercial Banks [Member]
Unsecured Debt			143,700,000	143,700,000	99,600,000
Commercial Banks [Member] | Unsecured Credit Facility [Member]
Unsecured Debt			44,000,000.0	44,000,000.0	0
Senior Unsecured Notes [Member]
Unsecured Debt			2,380,400,000	2,380,400,000	1,736,800,000
Secured Notes Fixed Rate Debt due 2019 [Member] | Secured Debt [Member]
Repayments of Secured Debt				439,300,000
2.70% Term loan Due 2022 | Commercial Banks [Member]
Unsecured Debt			$ 99,700,000	$ 99,700,000	$ 99,600,000
Debt Instrument, Maturity Date				Jan. 01, 2022
Notes payable, effective interest rate			2.70%	2.70%	3.34%
2.70% Term loan Due 2022 | Unsecured Debt [Member]
Debt Instrument, Face Amount								$ 100,000,000.0
3.74% Notes Due 2028 | Senior Unsecured Notes [Member]
Unsecured Debt			$ 396,700,000	$ 396,700,000	$ 396,100,000
Debt Instrument, Maturity Date				Oct. 15, 2028
Notes payable, effective interest rate			3.74%	3.74%	3.74%
Unsecured Credit Facility [Member]
Value not exceeding the amount available under the unsecured credit facility				$ 450,000,000
Amount of current borrowing capacity under unsecured credit facility			$ 600,000,000	600,000,000
Line of Credit Facility, Maximum Borrowing Capacity			900,000,000	$ 900,000,000
Terms Of Bid Rate Loans				P180D
Floating Rate Notes Payable [Member]
Total notes payable			143,700,000	$ 143,700,000	$ 99,600,000
Weighted average interest rate on floating rate debt									3.30%
3.67% Notes due 2029 | Senior Unsecured Notes [Member]
Debt Instrument, Face Amount		$ 600,000,000.0
Unsecured Debt			$ 593,700,000	$ 593,700,000	$ 0
Debt Instrument, Maturity Date				Jul. 01, 2029
Notes payable, effective interest rate		3.179%	3.67%	3.67%	0.00%
Debt Instrument, Interest Rate, Stated Percentage		3.15%
Discounted notes payable face amount		99.751%
Derivative, Notional Amount		$ 300,000,000.0
Debt Instrument, Interest Rate Before June 2026				3.84%
Debt Instrument, Interest Rate After June 2026				3.28%
Debt Instrument, Interest Rate During Period				3.67%
Proceeds from Issuance of Debt		$ 593,400,000
3.41% Notes due 2049 | Senior Unsecured Notes [Member]
Debt Instrument, Face Amount							$ 300,000,000.0
Unsecured Debt			$ 296,600,000	$ 296,600,000	$ 0
Debt Instrument, Maturity Date				Nov. 01, 2049
Notes payable, effective interest rate			3.41%	3.41%	0.00%		3.353%
Debt Instrument, Interest Rate, Stated Percentage							3.35%
Discounted notes payable face amount				99.941%
Debt Instrument, Interest Rate During Period				3.41%
Proceeds from Issuance of Debt				$ 296,600,000
4.78% Notes Due 2021 | Senior Unsecured Notes [Member]
Unsecured Debt			$ 0	$ 0	$ 249,100,000		$ 250,000,000
Debt Instrument, Maturity Date				Jun. 15, 2021
Notes payable, effective interest rate					4.78%		4.78%
Secured Notes 4.38% Fixed Rate Note Due 2045 [Member] | Secured Debt [Member]
Secured Debt							$ 45,300,000
Secured Notes 4.38% Fixed Rate Mortgage Note Due 2045 [Member] | Secured Debt [Member]
Secured Debt			$ 0	$ 0	$ 45,900,000
Debt Instrument, Maturity Date				Sep. 01, 2045
Notes payable, effective interest rate			0.00%	0.00%	4.38%

[1] Includes the effects of the applicable settled forward interest rate swaps.