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Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts, Claims Development by Accident Year (Details) $ in Millions | Mar. 31, 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 | $ 4,694.6 |  |  |  |  |
Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 3,998.2 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 1,952.8 |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 2,045.4 |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 211.2 |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,256.6 |  |  |  |  |
Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 3,175.6 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 1,447.8 |  |  |  |  |
All outstanding liabilities for 2012 and subsequent years, net of reinsurance | 1,727.8 |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 710.2 |  |  |  |  |
Liability for claims and claims adjustment expenses, net of reinsurance | 2,438.0 |  |  |  |  |
Accident Year 2012 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 644.7 | $ 649.0 | $ 664.1 | $ 650.5 | $ 621.6 | $ 599.2
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 58.1 |  |  |  |  |
Number of Reported Claims | Claim | 15,513 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 505.1 | 497.9 | 448.4 | 374.9 | 305.1 | 115.7
Accident Year 2012 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 681.0 | 683.6 | 684.8 | 714.3 | 730.9 | 686.3
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 103.3 |  |  |  |  |
Number of Reported Claims | Claim | 3,566 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 471.2 | 454.6 | 410.9 | 357.4 | 245.3 | $ 62.7
Accident Year 2013 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 659.7 | 654.6 | 673.4 | 662.2 | 683.5 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 98.5 |  |  |  |  |
Number of Reported Claims | Claim | 14,415 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 459.6 | 451.0 | 361.7 | 252.0 | 89.5 |
Accident Year 2013 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 510.9 | 515.2 | 540.7 | 561.8 | 569.6 |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 108.2 |  |  |  |  |
Number of Reported Claims | Claim | 3,356 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 321.6 | 313.5 | 272.3 | 181.9 | $ 62.1 |
Accident Year 2014 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 701.5 | 696.4 | 723.9 | 749.2 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 153.5 |  |  |  |  |
Number of Reported Claims | Claim | 18,459 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 400.7 | 386.8 | 245.5 | 98.9 |  |
Accident Year 2014 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 499.4 | 506.9 | 522.5 | 543.4 |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 142.1 |  |  |  |  |
Number of Reported Claims | Claim | 3,183 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 255.7 | 244.9 | 169.8 | $ 56.3 |  |
Accident Year 2015 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 879.5 | 895.3 | 906.1 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 261.7 |  |  |  |  |
Number of Reported Claims | Claim | 20,271 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 386.9 | 322.9 | 118.0 |  |  |
Accident Year 2015 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 539.9 | 556.2 | 573.5 |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 197.0 |  |  |  |  |
Number of Reported Claims | Claim | 3,088 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 210.6 | 168.8 | $ 56.7 |  |  |
Accident Year 2016 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 919.1 | 899.0 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 417.6 |  |  |  |  |
Number of Reported Claims | Claim | 18,879 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 197.3 | 116.0 |  |  |  |
Accident Year 2016 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 768.0 | 741.1 |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 406.4 |  |  |  |  |
Number of Reported Claims | Claim | 2,415 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 177.1 | $ 124.9 |  |  |  |
Accident Year 2017 | Insurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 193.7 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 165.0 |  |  |  |  |
Number of Reported Claims | Claim | 2,561 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 3.2 |  |  |  |  |
Accident Year 2017 | Reinsurance |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expense, Net | 176.4 |  |  |  |  |
Incurred but Not Reported (IBNR) Claims Liability, Net | $ 141.6 |  |  |  |  |
Number of Reported Claims | Claim | 849,000,000 |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 11.6 |  |  |  |  |

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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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