Clearing Operations (Schedule of Derivative Contracts Outstanding) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014 contract		Mar. 31, 2013 contract
Clearing Operations [Line Items]
Market value of derivative contracts	$ 2,926
Total Number of Cleared Contracts	25,179,779		27,882,196
Commodity forwards and options [Member]
Clearing Operations [Line Items]
Market value of derivative contracts	2,217	[1],[2]
Total Number of Cleared Contracts	246,134	[3]	236,276	[3]
Fixed-income options and futures [Member]
Clearing Operations [Line Items]
Market value of derivative contracts	332	[2],[4]
Total Number of Cleared Contracts	5,842,527		8,827,632
Stock options and futures [Member]
Clearing Operations [Line Items]
Market value of derivative contracts	176	[2],[4]
Total Number of Cleared Contracts	8,965,181		8,487,497
Index options and futures [Member]
Clearing Operations [Line Items]
Market value of derivative contracts	$ 201	[2],[4]
Total Number of Cleared Contracts	10,125,937		10,330,791

[1] We determined the fair value of our forward contracts using standard valuation models that were based on market-based observable inputs including LIBOR rates and the spot price of the underlying instrument.

[2] We determined the fair value of our option contracts using standard valuation models that were based on market-based observable inputs including implied volatility, interest rates and the spot price of the underlying instrument.

[3] The total volume in cleared power related to commodity contracts was 429 Terawatt hours (TWh) for the three months ended March 31, 2014 and 460 TWh for the three months ended March 31, 2013.

[4] We determined the fair value of our futures contracts based upon quoted market prices and average quoted market yields.