Borrowings Under Credit Facilities and Related Items (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016		Dec. 31, 2016
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 420,000	[1]	$ 1,350,000	[1]	$ 420,000	[1]	$ 1,350,000	[1]	$ 645,000
Maximum amount outstanding at any month end	645,000		1,560,000		1,010,000		1,560,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 450,130		$ 1,050,217		$ 601,436		$ 782,427
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	2.19%		1.44%		1.95%		1.35%

[1] As of September 30, 2017, letters of credit in the aggregate amount of $25,576,000 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.