Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2019	Jun. 06, 2019	Dec. 31, 2018
Notes payable, effective interest rate			3.179%
Unsecured notes payable		$ 2,432,137		$ 1,836,427
Secured notes payable		45,250		485,176
Total notes payable (1)		2,477,400		2,321,600
Senior Unsecured Notes [Member]
Unsecured notes payable	[1]	2,332,400		1,736,800
Secured Debt [Member]
Secured notes payable		$ 45,300		$ 485,200
3.11% Term loan, due 2022 [Member] | Commercial Banks [Member]
Debt Instrument, Maturity Date		Jan. 01, 2022
Notes payable, effective interest rate		3.11%		3.11%
Unsecured notes payable		$ 99,700		$ 99,600
4.78% Notes Due 2021 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Jun. 15, 2021
Notes payable, effective interest rate				4.78%
Unsecured notes payable		$ 249,300		$ 249,100
3.15% Notes Due 2022 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Dec. 15, 2022
Notes payable, effective interest rate		3.15%		3.15%
Unsecured notes payable		$ 347,800		$ 347,300
5.07% Notes Due 2023 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Jun. 15, 2023
Notes payable, effective interest rate		5.07%		5.07%
Unsecured notes payable		$ 248,300		$ 248,000
4.36% Notes Due 2024 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Jan. 15, 2024
Notes payable, effective interest rate		4.36%		4.36%
Unsecured notes payable		$ 248,900		$ 248,700
3.68% Notes Due 2024 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Sep. 15, 2024
Notes payable, effective interest rate		3.68%		3.68%
Unsecured notes payable		$ 247,900		$ 247,600
3.74% Notes Due 2028 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Oct. 15, 2028
Notes payable, effective interest rate		3.74%		3.74%
Unsecured notes payable		$ 396,600		$ 396,100
3.67% Notes due 2029 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable		$ 593,600		$ 0
4.38% Conventional Mortgage Notes, due 2045 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		Sep. 01, 2045
Notes payable, effective interest rate				4.38%
Secured notes payable		$ 45,300		$ 45,900
5.19% Conventional Mortgage Notes, due 2019 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		May 01, 2019
Notes payable, effective interest rate		0.00%		5.19%
Secured notes payable		$ 0		$ 419,900
5.33% Conventional Mortgage Notes Due 2019 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		Jun. 01, 2019
Notes payable, effective interest rate		0.00%		5.33%
Secured notes payable		$ 0		$ 19,400
Unsecured 3.11% Floating Rate Debt [Member] | Commercial Banks [Member]
Notes payable, effective interest rate		3.11%

[1] Unamortized debt discounts and debt issuance costs of $18.0 million and $13.9 million are included in senior unsecured and secured notes payable as of September 30, 2019 and December 31, 2018, respectively.