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Investments (Details 4) (USD $) In Thousands, unless otherwise specified | 6 Months Ended
--- | ---
Jun. 30, 2012 | Jun. 30, 2011
Summary of amounts included in net realized gains (losses) and other-than-temporary impairments |  |
Balance as of January 1 | $ 201,317 | $ 139,766
Additions for credit impairments recognized on: |  |
Securities not previously impaired | 466 | 18,433
Securities previously impaired | 4,933 | 858
Reductions for credit impairments previously recognized on: |  |
Securities that matured or were sold during the period | (24,007) |
Balance as of June 30 | $ 182,709 | $ 159,057

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