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Commitments and Contingent Liabilities - Narrative (Details) $ / shares in Units, £ in Millions |  |  | 9 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 27, 2017 USD ($) | Jun. 30, 2016 | Sep. 30, 2018 USD ($) | Jun. 30, 2019 $ / shares | Sep. 30, 2018 GBP (£) | Sep. 19, 2018 USD ($) | May 10, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 GBP (£) | Sep. 30, 2017 USD ($)
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  | $ 4,526,400,000 |  |  |  |  | $ 4,638,800,000 |  |
Other investments |  |  | 100,500,000 |  |  |  |  | 0 |  |
Assets held by insurance regulators |  |  | 494,200,000 |  |  |  |  | 458,700,000 |  |
Minimum capital required | £ |  |  |  |  | £ 0.4 |  |  |  | £ 0.4 |
Maximum borrowing capacity | $ 200,000,000 |  |  |  |  |  |  |  |  |
Line Of Credit Facility Increasable Capacity | $ 100,000,000 |  |  |  |  |  |  |  |  |
Long-term Line of Credit |  |  | 0 |  |  |  |  |  |  |
Line Of Credit Facility Tangible Net Worth Restrictions Minimum |  |  | $ 2,323,100,000 |  |  |  |  |  |  |
Line Of Credit Facility Tangible Net Worth Net Income Restriction |  |  | 25.00% |  |  |  |  |  |  |
Line Of Credit Facility Tangible Net Worth Cash Proceeds From Issuance Of Capital Stock Restriction |  |  | 25.00% |  |  |  |  |  |  |
Percentage Of Consolidated Leverage Ratio Permitted |  |  | 35.00% |  |  |  |  |  |  |
Letter of Credit |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  | $ 840,300,000 |  |  |  |  | 658,500,000 |  |
Citi Europe | Letter of Credit |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  |  |  |  | $ 550,000,000
Outstanding letters of credit |  |  | 444,200,000 |  |  |  |  | 449,400,000 |  |
LIBOR | Citi Europe | Letter of Credit |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Variable rate basis spread |  | 1.00% |  |  |  |  |  |  |  |
Limited Partner | Real estate funds |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  | $ 0 |  |  |  |  | $ 100,000,000 |  |
Other investments |  |  |  |  |  | $ 13,800,000 | $ 86,200,000 |  |  |
Subsequent Event |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Business acquisition, share price | $ / shares |  |  |  | $ 42.75 |  |  |  |  |  |

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