Notes Payable (Narrative) (Details) $ in Thousands		1 Months Ended		3 Months Ended		4 Months Ended	9 Months Ended
		May 31, 2017	Feb. 28, 2017 USD ($)	Sep. 30, 2017 USD ($) yr	Sep. 30, 2016 USD ($)	May 15, 2017 USD ($)	Sep. 30, 2017 USD ($) yr	Sep. 30, 2016 USD ($)	May 12, 2017 USD ($)	Dec. 31, 2016 USD ($)
Unamortized debt discounts and debt issuance costs				$ 13,000			$ 13,000			$ 15,700
Unsecured Debt				1,338,117			1,338,117			1,583,236
Available amount under unsecured credit facility				586,600			586,600
Notes payable				$ 2,204,200			$ 2,204,200			2,480,600
Weighted Average Interest Rate	[1]			4.30%			4.30%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr				4.5			4.5
Repayments of Notes Payable			$ 30,700
Loss on early retirement of debt				$ 0	$ 0		$ 323	$ 0
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility				50,000			50,000
Outstanding balance under credit facility				13,400			13,400
Short-term Debt [Member]
Maximum borrowing capacity under unsecured credit facility									$ 45,000
Debt Instrument, Basis Spread on Variable Rate		0.95%
Other Short-term Borrowings				0			0
Available amount under unsecured credit facility				45,000			$ 45,000
Debt Instrument, Maturity Date							May 14, 2018
Commercial Banks [Member] | Unsecured Credit Facility [Member]
Unsecured Debt				0			$ 0
Senior Unsecured Notes [Member]
Unsecured Debt	[2]			1,338,100			1,338,100			1,583,200
5.83% Notes Due 2017 [Member] | Senior Unsecured Notes [Member]
Unsecured Debt				$ 0			$ 0			$ 246,600
Notes payable, effective interest rate				5.83%			5.83%			5.83%
Debt Instrument, Maturity Date							May 15, 2017
Repayments of Unsecured Debt						$ 246,800
Unsecured Credit Facility [Member]
Unsecured Credit Facility, Current Borrowing Capacity				$ 600,000			$ 600,000
Unsecured Credit Facility, Expiration Date							Aug. 07, 2019
Optional extension period of unsecured credit facility							August 2020
Maximum borrowing capacity under unsecured credit facility				900,000			$ 900,000
Maximum term of bid rate loans (days)							180
Lesser of amount stated or the amount available under the unsecured credit facility							$ 300,000
Floating rate notes payable [Member]
Notes payable				$ 175,000	$ 206,100		$ 175,000	$ 206,100		$ 205,700
Weighted Average Interest Rate				1.90%	1.40%		1.90%	1.40%

[1] Includes all available extension options.

[2] 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.