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Investments (Details) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2012 | Dec. 31, 2011
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Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | $ 4,945,921 | $ 5,346,897
Gross Unrealized Gains | 548,581 | 627,588
Gross Unrealized Losses | 104,212 | 144,196
Estimated Fair Value | 5,390,290 | 5,830,289
Non - credit other - than - temporary Impairments | 9,802 | 20,907
Other short-term investments amortized cost basis | 1,009,868 | 783,015
Other short-term investments gross unrealized gain | 193 | 57
Other short-term investments gross unrealized losses | 2 | 1
Other Short-term Investments | 1,010,059 | 783,071
Other investments amortized cost basis | 100 | 100
Other Investments | 100 | 100
Investment other than available for sale securities pledged as collateral amortized cost basis | 5,955,889 | 6,130,012
Investment other than available for sale securities pledged as collateral gross unrealized gain | 548,774 | 627,645
Investment other than available for sale securities pledged as collateral gross unrealized losses | 104,214 | 144,197
Investment other than available for sale securities pledged as collateral | 6,400,449 | 6,613,460
Amortized cost basis of securities pledged as collateral | 286,115 | 261,958
Gross unrealized gains on securities pledged as collateral | 797 | 1,696
Gross unrealized losses on securities pledged as collateral | 252 | 124
Fair value of securities pledged as collateral | 286,660 | 263,530
Amortized Cost on Investment | 6,242,004 | 6,391,970
Gross Unrealized Gains On Investment | 549,571 | 629,341
Gross Unrealized Loss On Investment | 104,466 | 144,321
Estimated fair value on investments | 6,687,109 | 6,876,990
Municipal obligations [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | 1,768,713 | 1,858,493
Gross Unrealized Gains | 166,950 | 144,989
Gross Unrealized Losses | 80 | 483
Estimated Fair Value | 1,935,583 | 2,002,999
Corporate obligations [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | 1,040,010 | 1,087,629
Gross Unrealized Gains | 76,454 | 63,074
Gross Unrealized Losses | 13,769 | 23,203
Estimated Fair Value | 1,102,695 | 1,127,500
Foreign obligations [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | 65,089 | 89,951
Gross Unrealized Gains | 3,753 | 4,844
Estimated Fair Value | 68,842 | 94,795
US government obligations [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | 109,927 | 107,717
Gross Unrealized Gains | 1,507 | 3,847
Gross Unrealized Losses | 45 | 2
Estimated Fair Value | 111,389 | 111,562
Amortized cost basis of securities pledged as collateral | 285,349 | 259,401
Gross unrealized gains on securities pledged as collateral | 741 | 1,525
Gross unrealized losses on securities pledged as collateral | 252 | 124
Fair value of securities pledged as collateral | 285,838 | 260,802
U.S. agency obligations [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | 80,053 | 80,960
Gross Unrealized Gains | 4,647 | 5,911
Estimated Fair Value | 84,700 | 86,871
Residential Mortgage-backed Securities [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | 1,065,132 | 1,119,517
Gross Unrealized Gains | 249,879 | 350,816
Gross Unrealized Losses | 40,487 | 57,816
Estimated Fair Value | 1,274,524 | 1,412,517
Non - credit other - than - temporary Impairments | 9,802 | 20,907
Amortized cost basis of securities pledged as collateral | 766 | 2,557
Gross unrealized gains on securities pledged as collateral | 56 | 171
Fair value of securities pledged as collateral | 822 | 2,728
Collateralized debt obligations [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | 42,099 | 48,686
Gross Unrealized Gains | 481 | 142
Gross Unrealized Losses | 1,214 | 2,591
Estimated Fair Value | 41,366 | 46,237
Other asset-backed securities [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Amortized Cost | 774,898 | 953,944
Gross Unrealized Gains | 44,910 | 53,965
Gross Unrealized Losses | 48,617 | 60,101
Estimated Fair Value | 771,191 | 947,808
Short-term Investments [Member] |  |
Summary of amortized cost and estimated fair value of investments, excluding VIE investments |  |
Other short-term investments gross unrealized losses | $ 2 | $ 1

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