Borrowings Under Lines of Credit Arrangements and Related Items (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Aggregate borrowings under the unsecured lines of credit arrangements
Borrowings under unsecured lines of credit arrangements	[1]	$ 835,000	$ 0	$ 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.