Borrowings Under Lines of Credit Arrangements and Related Items (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Aggregate borrowings under the unsecured lines of credit arrangements
Balance outstanding at quarter 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.