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Borrowings Under Lines of Credit Arrangements and Related Items (Tables)
 12 Months Ended
Dec. 31, 2017
Borrowings Under Line of Credit Arrangement and Related Items (Tables) [Abstract]  
Aggregate borrowings under the unsecured lines of credit arrangements

The following information relates to aggregate borrowings under the primary unsecured revolving credit facility for the periods presented (dollars in thousands):

Year Ended December 31,
201720162015
Balance outstanding at year end(1)$719,000$645,000$835,000
Maximum amount outstanding at any month end$1,010,000$1,560,000$835,000
Average amount outstanding (total of daily
principal balances divided by days in period)$597,422$762,896$452,644
Weighted-average interest rate (actual interest
expense divided by average borrowings outstanding)2.02%1.39%1.17%
(1) As of December 31, 2017, letters of credit in the aggregate amount of $22,365,000 have been issued, which reduce the available borrowing capacity on our primary unsecured revolving credit facility.