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Reinsurance - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016
Concentration Risk [Line Items] |  |  |  |
Plus reinsurance recoverable on unpaid losses at period end | $ 1,767.1 | $ 1,515.2 | $ 1,369.5 | $ 560.7
Reinsurance recoverables, uncollateralized | $ 1,237.8 | $ 1,001.9 |  |
AM Best, A Rating | Standard & Poor's, A Plus Rating | Lloyd's |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration Risk, Percentage | 14.10% | 13.80% |  |
AM Best, A Plus Rating | Standard & Poor's, AA- Rating | Munich Re |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration Risk, Percentage | 16.80% | 17.00% |  |
AM Best, A Plus Rating | Standard & Poor's, AA- Rating | Scor Global P&C SE |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration Risk, Percentage | 8.90% |  |  |
AM Best, A Plus Rating | Standard & Poor's, A Plus Rating | Aioi Nissay Dowa |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration Risk, Percentage |  | 7.60% |  |

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