Borrowings Under Credit Facilities and Related Items (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016		Dec. 31, 2016
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 385,000	[1]	$ 745,000	[1]	$ 385,000	[1]	$ 745,000	[1]	$ 645,000
Maximum amount outstanding at any month end	640,000		745,000		1,010,000		945,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 561,626		$ 623,077		$ 678,343		$ 647,060
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.94%		1.26%		1.87%		1.28%

[1] As of June 30, 2017, letters of credit in the aggregate amount of $32,456,000 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.