Unsecured Lines Of Credit Payable - Revolving Credit Facility (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unsecured Debt [Abstract]
Total revolving credit facilities at December 31	$ 700,000	$ 600,000	$ 600,000
Borrowings outstanding	188,000	166,000	120,000
Weighted average daily borrowings during the year	230,934	179,633	214,962
Maximum daily borrowings during the year	$ 429,000	$ 252,000	$ 358,000
Weighted average interest rate during the year	2.96%	2.15%	1.52%
Weighted average interest rate on borrowings outstanding at December 31	3.5224%	2.54%	1.64%

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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