Notes Payable (Narrative) (Details) $ in Thousands			3 Months Ended		9 Months Ended
		Oct. 05, 2020 USD ($)	Sep. 30, 2020 USD ($) yr		Sep. 30, 2020 USD ($) yr		Oct. 01, 2020 USD ($)	Apr. 16, 2020 USD ($)	Dec. 31, 2019 USD ($)		Sep. 30, 2019 USD ($)
Notes Payable			$ 3,225,800		$ 3,225,800				$ 2,524,100
Unamortized debt discounts and debt issuance costs			24,200		24,200				19,900
Unsecured Debt			3,225,799		3,225,799				2,524,099
Available amount under unsecured credit facility			$ 889,800		$ 889,800
Weighted Average Interest Rate	[1],[2]		3.50%		3.50%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr			8.5		8.5
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility			$ 50,000		$ 50,000
Outstanding balance under credit facility			10,200		10,200
Commercial Banks [Member] | Unsecured Credit Facility [Member]
Unsecured Debt			0		0				44,000
Senior Unsecured Notes [Member]
Unsecured Debt	[3]		3,126,000		3,126,000				2,380,400
Unsecured Credit Facility [Member]
Maximum borrowing capacity under unsecured credit facility			900,000		$ 900,000
Maximum term of bid rate loans (days)					180 days
Lesser of amount stated or the amount available under the unsecured credit facility					$ 450,000
Floating rate notes payable [Member]
Notes Payable			$ 99,800	[4]	$ 99,800	[4]			143,700	[4]	$ 99,700
Weighted Average Interest Rate			1.20%		1.20%						3.10%
Unsecured notes 2.80% due 2030 [Member] [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Interest Rate During Period			2.91%
Proceeds from Issuance of Debt			$ 743,100
Debt Instrument, Face Amount								$ 750,000
Debt Instrument, Interest Rate, Stated Percentage								2.80%
Discounted notes payable face amount			99.929%
1.17% Term loan, due 2022 [Member] | Commercial Banks [Member]
Unsecured Debt			$ 99,800		$ 99,800				$ 99,700
Notes payable, effective interest rate			1.15%		1.15%				2.70%
Debt Instrument, Maturity Date					Jan. 01, 2022
Subsequent Event [Member] | 1.17% Term loan, due 2022 [Member] | Commercial Banks [Member]
Unsecured Debt							$ 40,000
Repayments of Unsecured Debt		$ 100,000

[1] Includes all available extension options.

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

[3] Unamortized debt discounts and debt issuance costs of $24.2 million and $19.9 million are included in senior unsecured and secured notes payable as of September 30, 2020 and December 31, 2019, respectively.

[4] Includes balances outstanding under our unsecured credit facility at December 31, 2019.