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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, 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 | $ 4,749.0 |  |  |  |  |
Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 4,217.3 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 2,129.6 |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 2,087.7 |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 232.0 |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,319.7 |  |  |  |  |
Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 3,369.3 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 1,617.6 |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 1,751.7 |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 677.6 |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,429.3 |  |  |  |  |
Accident Year 2012 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 641.2 | $ 653.2 | $ 668.4 | $ 654.4 | $ 625.3 | $ 603.4
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 53.8 |  |  |  |  |
Number of Reported Claims | Claim | 15,541 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 519.3 | 502.8 | 453.2 | 378.7 | 307.5 | 116.8
Accident Year 2012 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 683.6 | 688.8 | 690.1 | 719.6 | 736.0 | 691.5
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 101.2 |  |  |  |  |
Number of Reported Claims | Claim | 3,620 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 483.4 | 458.0 | 414.1 | 360.3 | 247.6 | $ 63.2
Accident Year 2013 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 674.0 | 658.8 | 677.5 | 666.3 | 688.0 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 72.2 |  |  |  |  |
Number of Reported Claims | Claim | 14,461 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 473.4 | 455.2 | 364.0 | 253.8 | 90.4 |
Accident Year 2013 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 514.9 | 522.9 | 548.8 | 570.0 | 577.5 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 100.5 |  |  |  |  |
Number of Reported Claims | Claim | 3,457 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 342.4 | 318.8 | 277.2 | 185.7 | $ 63.0 |
Accident Year 2014 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 717.2 | 700.0 | 727.8 | 753.0 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 152.2 |  |  |  |  |
Number of Reported Claims | Claim | 18,577 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 423.6 | 389.6 | 247.0 | 99.4 |  |
Accident Year 2014 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 496.0 | 513.1 | 528.8 | 549.1 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 127.7 |  |  |  |  |
Number of Reported Claims | Claim | 3,316 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 271.9 | 247.6 | 172.0 | $ 56.8 |  |
Accident Year 2015 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 889.9 | 901.3 | 911.5 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 245.7 |  |  |  |  |
Number of Reported Claims | Claim | 20,566 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 427.5 | 325.6 | 118.8 |  |  |
Accident Year 2015 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 553.5 | 563.2 | 579.7 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 185.3 |  |  |  |  |
Number of Reported Claims | Claim | 3,299 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 236.5 | 171.1 | $ 57.3 |  |  |
Accident Year 2016 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 889.3 | 905.2 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 351.4 |  |  |  |  |
Number of Reported Claims | Claim | 19,623 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 265.4 | 116.9 |  |  |  |
Accident Year 2016 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 754.7 | 749.5 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 325.9 |  |  |  |  |
Number of Reported Claims | Claim | 2,764 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 249.4 | $ 125.9 |  |  |  |
Accident Year 2017 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 405.7 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 305.1 |  |  |  |  |
Number of Reported Claims | Claim | 5,979 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 20.4 |  |  |  |  |
Accident Year 2017 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 366.6 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 258.3 |  |  |  |  |
Number of Reported Claims | Claim | 1,242 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 34.0 |  |  |  |  |

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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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