Borrowings Under Lines of Credit Arrangements and Related Items (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Aggregate borrowings under the unsecured lines of credit arrangements
Borrowings under unsecured lines of credit arrangements	[1]	$ 645,000	$ 835,000	$ 0
Maximum amount outstanding at any month end		1,560,000	835,000	637,000
Average amount outstanding (total of daily principal balances divided by days in period)		$ 762,896	$ 452,644	$ 207,452
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)		1.39%	1.17%	1.50%

[1] As of December 31, 2016, letters of credit in the aggregate amount of $41,878,000 have been issued, which reduces the available borrowing capacity on the primary unsecured revolving credit facility.