Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Unsecured notes payable		$ 1,338,117	$ 1,583,236
Secured notes payable	[1]	866,134	897,352
Total notes payable		2,204,200	2,480,600
Senior Unsecured Notes [Member]
Unsecured notes payable	[1]	$ 1,338,100	$ 1,583,200
5.83% Notes Due 2017 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		May 15, 2017
Notes payable, effective interest rate		5.83%	5.83%
Unsecured notes payable		$ 0	$ 246,600
4.78% Notes Due 2021 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Jun. 15, 2021
Notes payable, effective interest rate		4.78%	4.78%
Unsecured notes payable		$ 248,600	$ 248,400
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		$ 346,500	$ 346,000
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		$ 247,500	$ 247,200
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		$ 248,400	$ 248,200
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		$ 247,100	$ 246,800
Secured Notes 1.87% to 5.77% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Secured notes payable		866,100	866,700
Tax-exempt Mortgage Note [Member] | Secured Debt [Member]
Secured notes payable		0	30,700
Secured Notes 1.87% Floating Rate [Member] | Secured Debt [Member]
Secured notes payable		$ 175,000	$ 175,000
Minimum [Member] | Secured Notes 1.87% to 5.77% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		Oct. 01, 2018
Notes payable, effective interest rate		1.87%
Minimum [Member] | Secured Notes 1.87% Floating Rate [Member] | Secured Debt [Member]
Notes payable, effective interest rate		1.87%
Maximum [Member] | Secured Notes 1.87% to 5.77% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		Sep. 01, 2045
Notes payable, effective interest rate		5.77%
Unsecured Credit Facility [Member] | Commercial Banks [Member]
Unsecured notes payable		$ 0
Short-term Debt [Member]
Debt Instrument, Maturity Date		May 14, 2018
Other Short-term Borrowings		$ 0

[1] Unamortized debt discounts and debt issuance costs of $13.0 million and $15.7 million are included in senior unsecured and secured notes payable as of September 30, 2017 and December 31, 2016, respectively.