Borrowings Under Credit Facilities and Related Items (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016		Mar. 31, 2015		Dec. 31, 2015
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 645,000	[1]	$ 410,000	[1]	$ 835,000
Maximum amount outstanding at any month end	945,000		430,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 671,044		$ 408,944
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.29%		1.21%

[1] (1) As of March 31, 2016, letters of credit in the aggregate amount of $48,930,000 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.