Borrowings Under Credit Facilities and Related Items (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 410,000,000	[1]	$ 562,000,000	[1]	$ 0
Maximum amount outstanding at any month end	430,000,000		562,000,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 408,944,000		$ 286,889,000
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.21%		1.34%

[1] (1) As of March 31, 2015, letters of credit in the aggregate amount of $65,335,000 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.