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Clearing Operations (Schedule of Derivative Contracts) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) TWh shares	Dec. 31, 2017 TWh shares
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 2,202
Total number of cleared contracts | shares	98,506,687	94,152,671
Total volume in cleared power, in Terawatt hours (TWh) | TWh	1,067	1,199
Commodity and seafood options, futures and forwards
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 1,196
Total number of cleared contracts | shares	1,649,912	2,824,188
Fixed-income options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 600
Total number of cleared contracts | shares	22,839,794	20,376,383
Stock options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 271
Total number of cleared contracts | shares	24,978,684	26,023,816
Index options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 135
Total number of cleared contracts | shares	49,038,297	44,928,284

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Clearing Operations [Line Items]

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Total volume in cleared power.

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The number of derivative instruments of a particular group held by the entity.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

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