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Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Investment securities |  |  |
Fair value unfunded commitments | $ 5.3 |  |
Private equity funds |  |  |
Investment securities |  |  |
Other long-term investments | 125.2 |  |
Number of Investments Held | 38 |  |
Hedge funds |  |  |
Investment securities |  |  |
Other long-term investments | 114.6 |  |
Number of Investments Held | 16 |  |
Total hedge and private equity funds included in other long-term investments |  |  |
Investment securities |  |  |
Other long-term investments | 239.8 |  | 254.2
Fair value unfunded commitments | 119.4 | [1] | 81.5
Other Long-term Investments Largest Single Investment | 16.0 |  | 27.4
Hedge funds |  |  |
Investment securities |  |  |
Other long-term investments | 114.6 |  | 125.3
Fair value unfunded commitments | 0 | [1] | 0
Hedge funds | Long/short equity |  |  |
Investment securities |  |  |
Other long-term investments | 60.3 |  | 48.8
Fair value unfunded commitments | 0 | [1] | 0
Hedge funds | Long/short credit & distressed |  |  |
Investment securities |  |  |
Other long-term investments | 22.7 |  | 32.3
Fair value unfunded commitments | 0 | [1] | 0
Hedge funds | Long diversified strategies |  |  |
Investment securities |  |  |
Other long-term investments | 1.7 |  | 16.9
Fair value unfunded commitments | 0 | [1] | 0
Hedge funds | Long/short equity REIT |  |  |
Investment securities |  |  |
Other long-term investments | 16.0 |  | 14.5
Fair value unfunded commitments | 0 | [1] | 0
Hedge funds | Long/short equity activist |  |  |
Investment securities |  |  |
Other long-term investments | 13.6 |  | 12.3
Fair value unfunded commitments | 0 | [1] | 0
Hedge funds | Long bank loan |  |  |
Investment securities |  |  |
Other long-term investments | 0.3 |  | 0.5
Fair value unfunded commitments | 0 | [1] | 0
Private equity funds |  |  |
Investment securities |  |  |
Other long-term investments | 125.2 |  | 128.9
Fair value unfunded commitments | 119.4 | [1] | 81.5
Private equity funds | Multi-sector |  |  |
Investment securities |  |  |
Other long-term investments | 23.3 |  | 26.9
Fair value unfunded commitments | 5.4 | [1] | 8.2
Private equity funds | Energy infrastructure & services |  |  |
Investment securities |  |  |
Other long-term investments | 36.3 |  | 28.0
Fair value unfunded commitments | 15.6 | [1] | 9.9
Private equity funds | Distressed residential real estate |  |  |
Investment securities |  |  |
Other long-term investments | 15.8 |  | 27.4
Fair value unfunded commitments | 0 | [1] | 0
Private equity funds | Real estate |  |  |
Investment securities |  |  |
Other long-term investments | 11.6 |  | 9.5
Fair value unfunded commitments | 3.3 | [1] | 3.3
Private equity funds | Private equity secondaries |  |  |
Investment securities |  |  |
Other long-term investments | 10.5 |  | 11.3
Fair value unfunded commitments | 3.1 | [1] | 4.0
Private equity funds | International multi-sector, Europe |  |  |
Investment securities |  |  |
Other long-term investments | 5.1 |  | 7.8
Fair value unfunded commitments | 5.0 | [1] | 4.7
Private equity funds | Manufacturing/Industrial |  |  |
Investment securities |  |  |
Other long-term investments | 9.9 |  | 6.2
Fair value unfunded commitments | 15.1 | [1] | 0
Private equity funds | Healthcare |  |  |
Investment securities |  |  |
Other long-term investments | 4.3 |  | 2.3
Fair value unfunded commitments | 5.4 | [1] | 7.0
Private equity funds | International multi-sector, Asia |  |  |
Investment securities |  |  |
Other long-term investments | 0.4 |  | 3.6
Fair value unfunded commitments | 2.7 | [1] | 2.6
Private equity funds | Insurance |  |  |
Investment securities |  |  |
Other long-term investments | 3.0 |  | 3.5
Fair value unfunded commitments | 41.3 | [1] | 41.3
Private equity funds | Aerospace/Defense/Government |  |  |
Investment securities |  |  |
Other long-term investments | 2.8 |  | 0
Fair value unfunded commitments | 22.2 | [1] | 0
Private equity funds | Venture capital |  |  |
Investment securities |  |  |
Other long-term investments | 2.2 |  | 2.4
Fair value unfunded commitments | $ 0.3 | [1] | $ 0.5
[1] | White Mountains also has unfunded commitments to a tax advantaged federal affordable housing development fund of $5.3 at December 31, 2012.

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- Definition The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=SL6742756-110258 + Details
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- Definition This element represents the total fair market value of investments in certain entities that calculate net asset value per share (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements, but may also be used in both the balance sheet and disclosure in the same submission. This item represents alternative investments as of the balance sheet date which may include investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds, and any other investment in which investee management calculates net asset value per share as the primary metric for investment value. + References No definition available. + Details
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