Short-Term Financing - Summary of Short Term Financing and Weighted Average Interest Rate on Borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Oustanding Balance	$ 415,747	$ 377,767
Commercial paper (secured)
Short-term Debt [Line Items]
Oustanding Balance	$ 275,962	$ 238,013
Weighted Average Interest Rate	1.64%	1.48%
Prime broker arrangement
Short-term Debt [Line Items]
Oustanding Balance	$ 139,785	$ 127,754
Weighted Average Interest Rate	0.92%	0.91%
Bank lines (secured)
Short-term Debt [Line Items]
Oustanding Balance	$ 0	$ 12,000
Weighted Average Interest Rate		1.50%