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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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