Notes Payable (Narrative) (Details) $ in Thousands		1 Months Ended	3 Months Ended
		May 31, 2017	Mar. 31, 2018 USD ($) yr	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	May 12, 2017 USD ($)
Unamortized debt discounts and debt issuance costs			$ 11,600		$ 12,300
Unsecured Debt			1,339,142		1,338,628
Available amount under unsecured credit facility			586,900
Notes payable			$ 2,204,900		2,204,600
Weighted Average Interest Rate	[1]		4.30%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr			4.0
Loss on early retirement of debt			$ 0	$ 323
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility			50,000
Outstanding balance under credit facility			13,100
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
Available amount under unsecured credit facility			$ 45,000
Debt Instrument, Maturity Date			May 14, 2018
Commercial Banks [Member] | Unsecured Credit Facility [Member]
Unsecured Debt			$ 0
Senior Unsecured Notes [Member]
Unsecured Debt	[2]		$ 1,339,100		$ 1,338,600
5.83% Notes Due 2017 [Member] | Senior Unsecured Notes [Member]
Notes payable, effective interest rate			5.83%		5.83%
Debt Instrument, Maturity Date			May 15, 2017
Unsecured Credit Facility [Member]
Unsecured Credit Facility, Current Borrowing Capacity			$ 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
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		$ 175,000
Weighted Average Interest Rate			2.20%		1.40%

[1] Includes all available extension options.

[2] Unamortized debt discounts and debt issuance costs of $11.6 million and $12.3 million are included in senior unsecured and secured notes payable as of March 31, 2018 and December 31, 2017, respectively.