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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, 2017 USD ($) Claim | Dec. 31, 2016 USD ($) | 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 | $ 5,075.5 |  |  |  |  |
Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 4,403.8 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 2,322.7 |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 2,081.1 |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 140.3 |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,221.4 |  |  |  |  |
Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 3,904.7 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 1,769.5 |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 2,135.2 |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 718.9 |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,854.1 |  |  |  |  |
Accident Year 2012 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 637.0 | $ 656.7 | $ 671.8 | $ 657.6 | $ 628.4 | $ 606.9
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 47.2 |  |  |  |  |
Number of Reported Claims | Claim | 15,825 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 519.2 | 504.3 | 454.1 | 379.5 | 308.6 | 117.6
Accident Year 2012 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 683.8 | 692.8 | 694.2 | 723.7 | 740.0 | 695.5
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 93.4 |  |  |  |  |
Number of Reported Claims | Claim | 3,660 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 491.0 | 460.6 | 416.3 | 362.2 | 249.3 | $ 63.6
Accident Year 2013 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 668.9 | 662.4 | 681.0 | 669.9 | 691.9 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 74.6 |  |  |  |  |
Number of Reported Claims | Claim | 14,728 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 503.1 | 456.2 | 365.9 | 255.3 | 91.1 |
Accident Year 2013 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 519.5 | 528.6 | 554.8 | 575.9 | 583.2 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 92.9 |  |  |  |  |
Number of Reported Claims | Claim | 3,524 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 354.7 | 322.6 | 280.7 | 188.2 | $ 63.6 |
Accident Year 2014 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 686.2 | 704.8 | 732.7 | 758.1 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 109.7 |  |  |  |  |
Number of Reported Claims | Claim | 18,848 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 448.9 | 390.7 | 248.2 | 99.8 |  |
Accident Year 2014 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 496.7 | 517.9 | 533.8 | 553.5 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 116.8 |  |  |  |  |
Number of Reported Claims | Claim | 3,440 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 281.8 | 249.6 | 173.5 | $ 57.2 |  |
Accident Year 2015 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 859.6 | 909.4 | 919.5 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 206.4 |  |  |  |  |
Number of Reported Claims | Claim | 21,142 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 459.5 | 328.0 | 119.5 |  |  |
Accident Year 2015 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 553.5 | 568.6 | 584.8 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 166.5 |  |  |  |  |
Number of Reported Claims | Claim | 3,450 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 254.1 | 172.8 | $ 57.7 |  |  |
Accident Year 2016 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 876.3 | 912.7 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 316.0 |  |  |  |  |
Number of Reported Claims | Claim | 20,538 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 332.1 | 117.7 |  |  |  |
Accident Year 2016 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 763.8 | 756.5 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 290.4 |  |  |  |  |
Number of Reported Claims | Claim | 3,032 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 297.4 | $ 127.0 |  |  |  |
Accident Year 2017 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 675.8 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 426.5 |  |  |  |  |
Number of Reported Claims | Claim | 10,694 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 59.9 |  |  |  |  |
Accident Year 2017 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 887.4 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 636.9 |  |  |  |  |
Number of Reported Claims | Claim | 1,704 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 90.5 |  |  |  |  |

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- Definition Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671311-158438 + Details
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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671317-158438 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671321-158438 + Details
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