Performance Bonds and Guaranty Fund Contributions (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Guarantor Obligations [Line Items]
Cash and cash equivalents	$ 1,434.3	$ 1,366.1	$ 1,087.9	$ 2,469.7
Average Daily Clearing Settlement	4,100.0
US Treasury Securities [Member]
Guarantor Obligations [Line Items]
Cash and cash equivalents	15,100.0
Cash and Cash Equivalents, Fair Value Disclosure	15,100.0
Repurchase Agreements [Member]
Guarantor Obligations [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 1,800.0