Investments - Summary of Selected Financial Information for KCG (Details) - KCG Holdings, Inc [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Total assets		$ 6,040.5
Total liabilities		4,596.4
Total equity		$ 1,444.1
Subsequent event [Member]
Schedule of Equity Method Investments [Line Items]
Total assets	$ 6,260.8
Total liabilities	4,903.5
Total equity	$ 1,357.3