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Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Mar. 31, 2013 | Mar. 31, 2012 | Mar. 31, 2013 OneBeacon Runoff | Dec. 31, 2012 OneBeacon Runoff | Mar. 31, 2012 OneBeacon Runoff | Dec. 31, 2011 OneBeacon Runoff
Loss and Loss Adjustment Expense Reserve |  |  |  |  |  |  |  |  |  |
Gross beginning balance | $ 3,168.9 | $ 5,702.3 |  |  |  |  |  |  |  |
Less beginning reinsurance recoverable on unpaid losses | (429.1) | (2,507.3) |  |  |  |  |  |  |  |
Net loss and LAE reserves | 2,739.8 | 3,195.0 | 0 | [1] | 0 | [1] | 332.4 | [1] | 383.3 | [1]
Loss and LAE reserves acquired - American Fuji | 21.3 | 0 |  |  |  |  |  |  |  |
Loss and LAE incurred relating to current year losses | 242.9 | 257.4 |  |  |  |  |  |  |  |
Net favorable and unfavorable loss reserve development | 1.4 | (7.6) |  |  |  |  |  |  |  |
Total incurred losses and LAE | 244.3 | 249.8 |  |  |  |  |  |  |  |
Accretion of fair value adjustment to net loss and LAE reserves | 1.2 | 7.1 |  |  |  |  |  |  |  |
Foreign currency translation adjustment to net loss and LAE reserves | (9.6) | 11.8 |  |  |  |  |  |  |  |
Current year losses | (36.6) | (44.5) |  |  |  |  |  |  |  |
Prior year losses | (270.6) | (322.9) |  |  |  |  |  |  |  |
Total loss and LAE payments | (307.2) | (367.4) |  |  |  |  |  |  |  |
Net loss and LAE reserves | 2,689.8 | 3,045.4 | 0 | [1] | 0 | [1] | 332.4 | [1] | 383.3 | [1]
Plus ending reinsurance recoverable on unpaid losses | 410.8 | 2,465.4 |  |  |  |  |  |  |  |
Gross ending balance | $ 3,100.6 | $ 5,510.8 |  |  |  |  |  |  |  |
[1] | Loss and LAE reserve balances for OneBeacon's run-off business prior to September 30, 2012 were not classified as held for sale. Adjustment is to present loss and LAE reserve activities from continuing operations.

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