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Reserves for Losses and Adjustment Expenses Short-duration insurance contracts - Reconciliation of Claims Development (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | $ 4,815.5 |  |  |
Reinsurance recoverable on unpaid losses | 2,319.8 |  |  |
Unallocated claims incurred | 44.0 |  |  |
Other | (0.1) |  |  |
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items | (183.5) |  |  |
Losses and loss adjustment expenses | 6,951.8 | $ 7,074.2 | $ 6,749.5 | $ 5,319.9
Property Insurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 190.7 |  |  |
Casualty Insurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 669.2 |  |  |
Marine, Aviation and Energy Insurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 591.5 |  |  |
Financial and Professional Insurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 657.4 |  |  |
Insurance lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 2,108.8 |  |  |
Reinsurance recoverable on unpaid losses | 1,772.8 |  |  |
Property Catastrophe and Other Property Reinsurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 528.6 |  |  |
Casualty Reinsurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 1,602.2 |  |  |
Specialty Reinsurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 575.9 |  |  |
Reinsurance lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 2,706.7 |  |  |
Reinsurance recoverable on unpaid losses | 547.0 |  |  |
Insurance lines other than short-duration |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Other reinsurance balances recoverable | (227.4) |  |  |
Losses and loss adjustment expenses | $ 0.0 |  |  |

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- Definition Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Other Reconciling Item, Other Reinsurance Balances Recoverable + References No definition available. + Details
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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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