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Reinsurance and Indemnification - Michigan Catastrophic Claims Association (Details) - USD ($) | 6 Months Ended | 12 Months Ended | 24 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2018 | Dec. 31, 2018 | Jun. 30, 2021 | Jun. 30, 2019 | Dec. 31, 2017
Reinsurance Retention Policy [Line Items] |  |  |  |  |
Reinsurance and indemnification recoverables, net |  | $ 9,565,000,000 |  |  | $ 8,921,000,000
Assets |  | 112,249,000,000 |  |  | 112,422,000,000
Retained income | $ 2,920,000,000 | 44,033,000,000 |  |  | 41,579,000,000
Retained Earnings Accumulated Deficit Increase (Decrease) | (47,240,000,000) |  |  |  |
Michigan Catastrophic Claim Association |  |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |  |
Reinsurance and indemnification recoverables, net |  | $ 5,400,000,000 |  |  | $ 5,260,000,000
Reinsurance increase, percentage |  | 6.00% |  |  |
Program funding, per vehicle annual assessment |  | $ 192 |  |  |
Assets | $ 20,540,000,000 |  |  |  |
Scenario, Forecast | Michigan Catastrophic Claim Association |  |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |  |
Retention level per claim |  |  | $ 580,000 | $ 555,000 |
Term of catastrophe reinsurance agreement contract |  |  | 2 years | 2 years |

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