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Investments (Narrative) (Details) - USD ($)	1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2015	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Mar. 31, 2014
Investments, Debt and Securities [Line Items]
Trading securities		$ 202,000,000			$ 202,000,000		$ 202,000,000		$ 221,000,000
Gains and losses on available-for-sale investment securities							0		0
Net income from unconsolidated investees					6,000,000	$ 4,000,000	13,000,000	$ 10,000,000
Equity security impairment loss							0
Equity securities		76,000,000			76,000,000		76,000,000		$ 152,000,000
OCC
Investments, Debt and Securities [Line Items]
Committed equity contribution				$ 150,000,000
Capital contribution				$ 30,000,000
Annual dividend from shareholders percentage				50.00%
Shares owned (in shares)			2			2		2
Annual dividend received		13,000,000	$ 10,000,000
Borsa Istanbul Cost Method Investment
Investments, Debt and Securities [Line Items]
Ownership percentage									5.00%
Equity securities										$ 75,000,000
Foreign Government Debt Securities
Investments, Debt and Securities [Line Items]
Trading securities		$ 174,000,000			$ 174,000,000		$ 174,000,000		$ 160,000,000
LCH
Investments, Debt and Securities [Line Items]
Ownership percentage							5.00%		5.00%
LCH | Subsequent Event
Investments, Debt and Securities [Line Items]
Ownership percentage	5.00%

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