Notes Payable (Narrative) (Details) $ in Thousands		1 Months Ended		2 Months Ended	12 Months Ended
	Oct. 02, 2018 USD ($)	May 31, 2018	Aug. 31, 2015 USD ($)	Feb. 15, 2019 USD ($)	Dec. 31, 2018 USD ($) yr	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 04, 2018 USD ($)	Oct. 01, 2018 USD ($)	May 12, 2018 USD ($)
Secured Debt					$ 485,176	$ 865,970
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net					$ 13,900	12,300
Current Fiscal Year End Date					--12-31
Unsecured Debt					$ 1,836,427	1,338,628
Total notes payable					2,321,600	2,204,600
Loss on early retirement of debt					0	323	$ 0
Available amount under unsecured credit facility					$ 589,900
Weighted average interest rate on floating rate debt					4.20%
Weighted average maturity of indebtedness (including unsecured credit facility and unsecured short-term borrowings) (in years) | yr					4.9
Short-term Debt [Member]
Other Short-term Borrowings					$ 0
Maximum borrowing capacity under unsecured credit facility										$ 45,000
Debt Instrument, Maturity Date					May 14, 2018
Debt Instrument, Basis Spread on Variable Rate		0.95%
Available amount under unsecured credit facility					$ 45,000
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility					50,000
Outstanding balance under credit facility					10,100
Senior Unsecured Notes [Member]
Unsecured Debt					1,736,800	1,338,600
Term loan Due 2022 | Unsecured Debt [Member]
Debt Instrument, Face Amount									$ 100,000
Unsecured Debt					99,600	0
3.74% Notes Due 2028 | Unsecured Debt [Member]
Debt Instrument, Interest Rate, Stated Percentage	4.10%
Discounted notes payable face amount	99.893%
Debt Instrument, Face Amount	$ 400,000
Notes payable, effective interest rate	4.113%
Debt Instrument, Interest Rate, Basis for Effective Rate	0.0374
Proceeds from Issuance of Debt	$ 396,100
3.74% Notes Due 2028 | Senior Unsecured Notes [Member]
Unsecured Debt					$ 396,100	0
Debt Instrument, Maturity Date					Oct. 15, 2028
Notes payable, effective interest rate					3.74%
Secured Notes Due 2018 | Secured Debt [Member]
Repayments of Secured Debt	$ 380,000
Secured Notes 1.92% Floating Rate [Member] | Secured Debt [Member]
Secured Debt					$ 0	175,000		$ 175,000
Secured Notes 5.77% Floating Rate [Member] | Secured Debt [Member]
Secured Debt								$ 205,000
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
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					$ 99,600	$ 175,000
Weighted average interest rate on floating rate debt						1.90%
Subsequent Event [Member]
Repayments of Secured Debt				$ 200,000