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Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts, Claims Development by Accident Year (Details) $ in Millions | Sep. 30, 2018 USD ($) Claim | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($)
--- | --- | --- | --- | --- | --- | --- | ---
Claims Development [Line Items] |  |  |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | $ 4,917.5 |  |  |  |  |  |
Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 5,027.8 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 3,023.0 |  |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 2,004.8 |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 101.3 |  |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,106.1 |  |  |  |  |  |
Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 4,797.8 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 2,555.5 |  |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 2,242.3 |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 569.1 |  |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,811.4 |  |  |  |  |  |
Accident Year 2012 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 644.3 | $ 637.1 | $ 653.6 | $ 668.7 | $ 654.8 | $ 625.8 | $ 603.9
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 27.2 |  |  |  |  |  |
Number of Reported Claims | Claim | 15,726 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 530.4 | 523.8 | 502.0 | 452.1 | 377.9 | 307.5 | 117.1
Accident Year 2012 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 680.4 | 686.8 | 688.3 | 689.6 | 720.0 | 736.3 | 691.8
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 59.5 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,875 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 514.0 | 495.8 | 458.1 | 414.2 | 360.4 | 247.7 | $ 63.0
Accident Year 2013 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 651.7 | 649.4 | 659.2 | 677.9 | 666.7 | 688.3 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 34.4 |  |  |  |  |  |
Number of Reported Claims | Claim | 14,661 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 537.4 | 501.9 | 454.2 | 364.2 | 253.9 | 90.4 |
Accident Year 2013 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 497.8 | 503.8 | 523.1 | 548.8 | 571.0 | 578.6 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 59.0 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,737 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 376.4 | 359.1 | 319.4 | 277.8 | 186.4 | $ 63.1 |
Accident Year 2014 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 696.7 | 693.7 | 701.0 | 728.7 | 754.2 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 51.9 |  |  |  |  |  |
Number of Reported Claims | Claim | 18,872 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 507.8 | 465.1 | 389.2 | 247.1 | 99.4 |  |
Accident Year 2014 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 480.3 | 485.6 | 512.8 | 528.4 | 549.4 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 76.4 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,769 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 314.4 | 288.4 | 247.8 | 172.1 | $ 56.9 |  |
Accident Year 2015 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 864.4 | 852.8 | 903.6 | 913.8 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 124.8 |  |  |  |  |  |
Number of Reported Claims | Claim | 21,221 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 544.5 | 470.6 | 326.0 | 119.0 |  |  |
Accident Year 2015 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 537.8 | 556.6 | 562.7 | 578.5 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 103.7 |  |  |  |  |  |
Number of Reported Claims | Claim | 4,006 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 317.6 | 271.5 | 170.9 | $ 57.4 |  |  |
Accident Year 2016 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 885.7 | 879.6 | 907.3 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 264.7 |  |  |  |  |  |
Number of Reported Claims | Claim | 21,224 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 448.2 | 345.5 | 117.1 |  |  |  |
Accident Year 2016 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 764.3 | 761.3 | 747.8 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 190.5 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,901 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 420.7 | 350.5 | $ 125.5 |  |  |  |
Accident Year 2017 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 815.6 | 894.2 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 158.6 |  |  |  |  |  |
Number of Reported Claims | Claim | 21,367 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 400.2 | 167.5 |  |  |  |  |
Accident Year 2017 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 1,185.0 | 1,187.2 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 339.2 |  |  |  |  |  |
Number of Reported Claims | Claim | 4,165 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 573.4 | $ 227.7 |  |  |  |  |
Accident Year 2018 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 469.4 |  |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 289.8 |  |  |  |  |  |
Number of Reported Claims | Claim | 12,316 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 54.5 |  |  |  |  |  |
Accident Year 2018 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 652.2 |  |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 517.9 |  |  |  |  |  |
Number of Reported Claims | Claim | 1,971 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 39.0 |  |  |  |  |  |

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- Definition Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=99384497&loc=SL65671321-158438 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99384497&loc=SL65671317-158438 + Details
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