Borrowings Under Credit Facilities and Related Items (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2015		Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014	Dec. 31, 2014
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 350,000,000	[1]	$ 0	$ 350,000,000	[1]	$ 0	$ 0
Maximum amount outstanding at any month end	535,000,000		637,000,000	535,000,000		637,000,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 501,758,000		$ 375,824,000	$ 455,608,000		$ 331,602,000
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.13%		1.37%	1.17%		1.36%

[1] (1) As of June 30, 2015, letters of credit in the aggregate amount of $61,882,247 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.