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Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts, Claims Development by Accident Year (Details) $ in Millions | Jun. 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 ($)
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Claims Development [Line Items] |  |  |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | $ 4,830.5 |  |  |  |  |  |
Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 4,884.2 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 2,885.9 |  |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 1,998.3 |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 114.0 |  |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,112.3 |  |  |  |  |  |
Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 4,521.2 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 2,422.9 |  |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 2,098.3 |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 619.9 |  |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,718.2 |  |  |  |  |  |
Accident Year 2012 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 647.2 | $ 638.2 | $ 654.7 | $ 669.9 | $ 655.9 | $ 626.8 | $ 605.0
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 45.9 |  |  |  |  |  |
Number of Reported Claims | Claim | 15,682 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 531.7 | 524.7 | 502.8 | 452.9 | 378.5 | 307.9 | 117.3
Accident Year 2012 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 686.7 | 687.9 | 689.4 | 690.6 | 720.9 | 737.3 | 692.8
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 71.2 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,838 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 511.4 | 496.5 | 458.7 | 414.8 | 360.9 | 248.2 | $ 63.2
Accident Year 2013 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 651.1 | 650.6 | 660.4 | 679.0 | 667.9 | 689.6 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 32.4 |  |  |  |  |  |
Number of Reported Claims | Claim | 14,631 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 532.7 | 502.8 | 455.0 | 364.9 | 254.5 | 90.7 |
Accident Year 2013 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 499.7 | 504.9 | 524.3 | 550.2 | 572.2 | 579.6 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 66.2 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,689 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 371.6 | 359.9 | 320.1 | 278.4 | 186.8 | $ 63.2 |
Accident Year 2014 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 699.5 | 695.1 | 702.4 | 730.1 | 755.6 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 68.0 |  |  |  |  |  |
Number of Reported Claims | Claim | 18,836 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 481.0 | 465.9 | 389.8 | 247.5 | 99.5 |  |
Accident Year 2014 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 477.5 | 487.1 | 514.2 | 529.9 | 550.6 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 82.8 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,696 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 308.1 | 289.2 | 248.5 | 172.6 | $ 57.0 |  |
Accident Year 2015 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 858.1 | 855.1 | 905.9 | 916.0 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 154.4 |  |  |  |  |  |
Number of Reported Claims | Claim | 21,132 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 523.4 | 471.9 | 326.9 | 119.2 |  |  |
Accident Year 2015 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 536.9 | 558.2 | 564.5 | 580.3 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 117.6 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,899 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 306.2 | 272.1 | 171.4 | $ 57.5 |  |  |
Accident Year 2016 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 873.6 | 881.6 | 909.3 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 284.6 |  |  |  |  |  |
Number of Reported Claims | Claim | 21,052 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 415.4 | 346.1 | 117.3 |  |  |  |
Accident Year 2016 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 768.9 | 762.8 | 749.1 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 218.1 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,709 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 400.0 | 350.7 | $ 125.5 |  |  |  |
Accident Year 2017 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 847.7 | 896.1 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 175.9 |  |  |  |  |  |
Number of Reported Claims | Claim | 20,622 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 369.5 | 167.8 |  |  |  |  |
Accident Year 2017 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 1,189.0 | 1,190.2 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 420.5 |  |  |  |  |  |
Number of Reported Claims | Claim | 3,746 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 508.1 | $ 227.9 |  |  |  |  |
Accident Year 2018 | Insurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 307.0 |  |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 192.9 |  |  |  |  |  |
Number of Reported Claims | Claim | 6,929 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 32.2 |  |  |  |  |  |
Accident Year 2018 | Reinsurance |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 362.5 |  |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 289.9 |  |  |  |  |  |
Number of Reported Claims | Claim | 1,415 |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 17.5 |  |  |  |  |  |

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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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