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Borrowings Under Lines of Credit Arrangements and Related Items (Tables)
 12 Months Ended
Dec. 31, 2015
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,
201520142013
Balance outstanding at year end(1)$835,000$-$130,000
Maximum amount outstanding at any month end$835,000$637,000$1,019,050
Average amount outstanding (total of daily
principal balances divided by days in period)$452,644$207,452$488,842
Weighted-average interest rate (actual interest
expense divided by average borrowings outstanding)1.17%1.50%1.45%
(1) As of December 31, 2015, letters of credit in the aggregate amount of $54,925,000 have been issued which reduce the available borrowing capacity on the primary unsecured credit facility.