Borrowings Under Credit Facilities and Related Items (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017		Dec. 31, 2017
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 865,000	[1]	$ 522,000	[1]	$ 719,000
Maximum amount outstanding at any month end	865,000		1,010,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 364,111		$ 796,356
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	2.72%		1.81%

[1] As of March 31, 2018, letters of credit in the aggregate amount of $22,365,000 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.