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Municipal Bond Guarantee Insurance (Tables) | 12 Months Ended
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Dec. 31, 2017
Insurance [Abstract] |
Financial Guarantee Insurance Contracts, Premium Received over Contract Period | The following table presents a schedule of BAM’s future premium revenues as of December 31, 2017:
Millions |  | December 31, 2017
January 1, 2018 - March 31, 2018 |  | $ | 2.9 |
April 1, 2018 - June 30, 2018 |  | 2.9 |
July 1, 2018 - September 30, 2018 |  | 2.9 |
October 1, 2018 - December 31, 2018 |  | 2.8 |
|  | 11.5 |
2019 |  | 11.1 |
2020 |  | 10.7 |
2021 |  | 10.2 |
2022 |  | 9.7 |
2023 and thereafter |  | 83.6 |
Total gross unearned insurance premiums |  | $ | 136.8 |
Schedule of Insured Obligations | The following table presents a schedule of BAM’s insured obligations as of December 31, 2017 and 2016:
|  | December 31, 2017 |  | December 31, 2016
Contracts outstanding |  | 6,371 |  |  | 4,807 |
Remaining weighted average contract period (in years) |  | 10.9 |  |  | 10.8 |
Contractual debt service outstanding (in millions): |  |  |  |
Principal |  | $ | 42,090.6 |  |  | $ | 33,057.3 |
Interest |  | 21,057.1 |  |  | 16,396.6 |
Total debt service outstanding |  | $ | 63,147.7 |  |  | $ | 49,453.9 |
Gross unearned insurance premiums |  | $ | 136.8 |  |  | $ | 82.9 |
Schedule of Net Written Premiums | The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the years ended December 31, 2017, 2016 and 2015:
Millions |  | December 31, 2017 |  | December 31, 2016 |  | December 31, 2015
Gross written premiums |  | $ | 63.2 |  |  | $ | 38.6 |  |  | $ | 25.9 |
Assumed (ceded) written premiums |  | — |  |  | — |  |  | — |
Net written premiums |  | $ | 63.2 |  |  | $ | 38.6 |  |  | $ | 25.9 |

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- Definition Tabular disclosure of financial guarantee insurance contracts in which premiums are received over the contract period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290 + Details
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- Definition [Table Text Block] for Schedule of Municipal Guarantee Insured Obligations [Table] + References No definition available. + Details
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- Definition Schedule of Net Written Premiums [Table Text Block] + References No definition available. + Details
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