Notes Payable (Narrative) (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) yr	Dec. 31, 2015 USD ($)
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		$ 15,700	$ 18,600
Unsecured Debt		1,583,236	1,824,930
Total notes payable		$ 2,480,600	2,724,700
Weighted average interest rate on floating rate debt		4.40%
Weighted average maturity of indebtedness (including unsecured credit facility and unsecured short-term borrowings) (in years) | yr		4.9
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility		$ 50,000
Outstanding balance under credit facility		12,700
Senior Unsecured Notes [Member]
Unsecured Debt		1,583,200	1,580,900
Commercial Banks [Member]
Unsecured Debt		0	244,000
Commercial Banks [Member] | Short-term Debt [Member]
Unsecured Debt		0	19,000
Commercial Banks [Member] | Unsecured Credit Facility [Member]
Unsecured Debt		0	225,000
Unsecured Credit Facility [Member]
Value not exceeding the amount available under the unsecured credit facility		300,000
Amount of current borrowing capacity under unsecured credit facility	$ 500,000	600,000
Maximum borrowing capacity under unsecured credit facility		$ 900,000
Terms Of Bid Rate Loans		180
Available amount under unsecured credit facility		$ 587,300
Unsecured Credit Facility, Expiration Date	Sep. 22, 2015	Aug. 07, 2019
Option to extend the period of unsecured credit facility		August 2020
Floating Rate Notes Payable [Member]
Total notes payable		$ 205,700	$ 451,400
Weighted average interest rate on floating rate debt		1.40%	1.20%