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Investments in Unconsolidated Affiliates (Tables) | 12 Months Ended
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Dec. 31, 2017
Investments in and Advances to Affiliates [Table Text Block] | The following table presents the carrying values of investments in equity method eligible unconsolidated entities recorded within other long-term investments:
|  | December 31
Millions |  | 2017 |  | 2016
Equity method eligible private equity securities, at fair value |  | $ | 58.0 |  |  | $ | 55.6 |
Investments, accounted for under the equity method |  | 4.6 |  |  | 3.5 |
Total investments in equity method eligible unconsolidated entities |  | 62.6 |  |  | 59.1 |
Other unconsolidated investments (1) |  | 146.2 |  |  | 113.7 |
Total other long-term investments |  | $ | 208.8 |  |  | $ | 172.8 |
(1) Consists of other long-term investments that are not equity method eligible.
Affiliated Entity [Member] |
Equity Method Investments [Table Text Block] | The following table presents White Mountains’s investments in equity method eligible unconsolidated entities as of December 31, 2017 and 2016:
Investee |  | Ownership Interest |  | Instrument Held
Compare.com |  | 22% |  | Common shares
durchblicker |  | 45% |  | Common shares
OneTitle |  | 20% |  | Common shares
PassportCard |  | 50% |  | Common shares
Tuckerman Capital Fund III |  | 21% |  | Units
Total investments in equity method eligible unconsolidated entities |
Equity Method Investments [Table Text Block] | The following tables presents aggregated summarized financial information for White Mountains’s investments in equity method eligible unconsolidated entities:
|  | December 31,
Millions |  | 2017 |  | 2016
Balance sheet data (1): |  |  |  |
Total assets |  | $ | 75.4 |  |  | $ | 79.4 |
Total liabilities |  | 24.2 |  |  | 18.8 |
(1) Financial data submitted by investees is on a one-quarter lag.
|  | Year Ended December 31,
Millions |  | 2017 |  | 2016 |  | 2015
Income statement data (1): |  |  |  |  |  |
Revenues |  | $ | 60.0 |  |  | $ | 32.9 |  |  | $ | 12.9 |
Expenses |  | (66.8 | ) |  | (76.4 | ) |  | (69.9 | )
(1) Financial data submitted by investees is on a one-quarter lag.
Symetra |
Equity Method Investments [Table Text Block] | The following tables presents financial information for Symetra as of September 30, 2015:
|  | September 30,
Millions |  | 2015
Symetra balance sheet data: |  |  |
Total investments |  | $ | 32,409.2 |
Separate account assets |  | 885.9 |
Total assets |  | 34,962.8 |
Policyholder liabilities |  | 29,492.0 |
Long-term debt |  | 697.5 |
Separate account liabilities |  | 885.9 |
Total liabilities |  | 31,836.7 |
Common shareholders’ equity |  | 3,126.1 |
The following tables presents financial information for Symetra for the nine months ended September 30, 2015:
|  | Nine months ended September 30, 2015
Millions |
Symetra income statement data: |  |
Net premiums earned |  | $ | 539.3 |
Net investment income |  | 994.3 |
Total revenues |  | 1,605.9 |
Policy benefits |  | 1,143.7 |
Total expenses |  | 1,543.6 |
Net income |  | 89.6 |
Comprehensive net loss |  | (234.1 | )

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- Definition Tabular disclosure of equity method investments including, but not limited to, name of each investee or group of investments, percentage ownership, difference between recorded amount of an investment and the value of the underlying equity in the net assets, and summarized financial information. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
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- Definition Tabular disclosure of investments in and advances to affiliates. Does not include the tabular disclosure of the disaggregation of investments in and advances to affiliates across legal entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611322-123010 + Details
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