Fair Value Measurements Fair Value Measurement (Details) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Assets [Abstract]
Investments		$ 9,875	$ 15,351
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		1,306	794
Fair value measurements, Recurring and nonrecurring [Member]
Assets [Abstract]
Investments		9,582	15,028
Derivative contracts, at fair value (asset)	[1]	19	259
Assets, Fair Value Disclosure		9,601	15,287
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		1,306	794
Other liabilities		21	3
Derivative contracts at fair value (liability)	[2]	448	36
Financial and Nonfinancial Liabilities, Fair Value Disclosure		1,775	833
Fair value measurements, Recurring and nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets [Abstract]
Investments		8,956	14,447
Derivative contracts, at fair value (asset)	[1]	0	0
Assets, Fair Value Disclosure		8,956	14,447
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		1,251	794
Other liabilities		0	0
Derivative contracts at fair value (liability)	[2]	0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure		1,251	794
Fair value measurements, Recurring and nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets [Abstract]
Investments		382	289
Derivative contracts, at fair value (asset)	[1]	19	259
Assets, Fair Value Disclosure		401	548
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		55	0
Other liabilities		21	3
Derivative contracts at fair value (liability)	[2]	448	36
Financial and Nonfinancial Liabilities, Fair Value Disclosure		524	39
Fair value measurements, Recurring and nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets [Abstract]
Investments		244	292
Derivative contracts, at fair value (asset)	[1]	0	0
Assets, Fair Value Disclosure		244	292
Liabilities [Abstract]
Securities sold, not yet purchased, at fair value		0	0
Other liabilities		0	0
Derivative contracts at fair value (liability)	[2]	0	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure		$ 0	$ 0

[1] Amounts are classified within other assets in our condensed consolidated balance sheets.

[2] Amounts are classified within accrued expenses and other liabilities in our condensed consolidated balance sheets.