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Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |
Derivative Collateral Right to Reclaim | $ 30.6 |  | $ 73.2 |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period | 290.4 |  |  |  |  |
Change in fair value of derivatives, included in other revenues | (339.0) |  | 92.9 |  | 127.0 |
Ending balance | 98.3 |  | 290.4 |  |  |
Level 3 |  |  |  |  |  |
Changes in variable annuity (liabilities) |  |  |  |  |  |
Beginning balance | (768.5) |  | (610.2) |  | (380.7) |
Purchases | 0 |  | 0 |  | 0 |
Realized and unrealized gains (losses) | 327.0 |  | (158.3) |  | (229.5) |
Transfers in (out) | 0 |  | 0 |  | 0 |
Sales/settlements | 0 |  | 0 |  | 0 |
Ending balance | (441.5) |  | (768.5) |  | (610.2) |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period | 247.1 | [1] | 275.3 | [1] | 208.5 | [1]
Purchases | 6.1 | [1] | 5.0 | [1] | 19.4 | [1]
Change in fair value of derivatives, included in other revenues | (84.0) | [1] | 14.5 | [1] | 66.4 | [1]
Transfers in (out) | 0 | [1] | 0 | [1] | 0 | [1]
Sales/settlements | (28.7) | [1] | (47.7) | [1] | (19.0) | [1]
Ending balance | 140.5 | [1] | 247.1 | [1] | 275.3 | [1]
Level 2 |  |  |  |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period | 39.2 | [1],[2] | 72.2 | [1],[2] | 23.8 | [1],[2]
Purchases | 0 | [1],[2] | 0 | [1],[2] | 0 | [1],[2]
Change in fair value of derivatives, included in other revenues | (186.9) | [1],[2] | 67.7 | [1],[2] | 80.0 | [1],[2]
Transfers in (out) | 0 | [1],[2] | 0 | [1],[2] | 0 | [1],[2]
Sales/settlements | 127.2 | [1],[2] | (100.7) | [1],[2] | (31.6) | [1],[2]
Ending balance | (20.5) | [1],[2] | 39.2 | [1],[2] | 72.2 | [1],[2]
Level 1 |  |  |  |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period | 4.1 | [3] | 0 | [3] | (38.0) | [3]
Purchases | 0 | [3] | 0 | [3] | 0 | [3]
Change in fair value of derivatives, included in other revenues | (68.1) | [3] | 10.7 | [3] | (19.4) | [3]
Transfers in (out) | 0 | [3] | 0 | [3] | 0 | [3]
Sales/settlements | 42.3 | [3] | (6.6) | [3] | 57.4 | [3]
Ending balance | (21.7) | [3] | 4.1 | [3] | 0 | [3]
Fair value |  |  |  |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period | 290.4 | [4] | 347.5 | [4] | 194.3 | [4]
Purchases | 6.1 | [4] | 5.0 | [4] | 19.4 | [4]
Change in fair value of derivatives, included in other revenues | (339.0) | [4] | 92.9 | [4] | 127.0 | [4]
Transfers in (out) | 0 | [4] | 0 | [4] | 0 | [4]
Sales/settlements | 140.8 | [4] | (155.0) | [4] | 6.8 | [4]
Ending balance | 98.3 | [4] | 290.4 | [4] | 347.5 | [4]
Cash |  |  |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |
Derivative Collateral Right to Reclaim | $ 249.8 |  | $ 453.5 |  | $ 326.0 |
[1] | Consists of over-the-counter instruments.
[2] | Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.
[3] | Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.
[4] | In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments of , $393.6, $485.3 and $326.0 at December 31, 2012, 2011 and 2010 posted as collateral to its reinsurance counterparties.

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- Definition Amount of gain (loss) recognized in earnings, arising from liabilities measured at fair value on a recurring basis using unobservable inputs (level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 157 -Paragraph 32 -Subparagraph c(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Purchases that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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- Definition This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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