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Investments (Details 2) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2012 | Dec. 31, 2011
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Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position |  |
Less than Twelve Months, Fair Value | $ 526,576 | $ 635,964
Less than 12 Months, Gross Unrealized Loss | 9,545 | 30,309
Twelve Months of Longer, Fair Value | 467,620 | 516,538
12 Months or Longer, Gross Unrealized Loss | 94,919 | 114,011
Total, Fair Value | 994,196 | 1,152,502
Total, Gross Unrealized Loss | 104,464 | 144,320
Total, Gross Unrealized Loss | 2 | 1
Less than Twelve Months, Fair Value | 532,861 | 641,737
Less than 12 Months, Gross Unrealized Loss | 9,547 | 30,310
Twelve Months or Longer, Fair Value | 467,620 | 516,538
12 Months or Longer, Gross Unrealized Losses | 94,919 | 114,011
Total, Fair Value | 1,000,481 | 1,158,275
Total, Gross Unrealized Loss | 104,466 | 144,321
Municipal obligations [Member] |  |
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position |  |
Less than Twelve Months, Fair Value | 8,205 | 9,359
Less than 12 Months, Gross Unrealized Loss | 6 | 309
Twelve Months of Longer, Fair Value | 7,333 | 14,635
12 Months or Longer, Gross Unrealized Loss | 74 | 174
Total, Fair Value | 15,538 | 23,994
Total, Gross Unrealized Loss | 80 | 483
Corporate obligations [Member] |  |
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position |  |
Less than Twelve Months, Fair Value | 47,050 | 155,528
Less than 12 Months, Gross Unrealized Loss | 1,122 | 6,220
Twelve Months of Longer, Fair Value | 143,704 | 178,861
12 Months or Longer, Gross Unrealized Loss | 12,647 | 16,983
Total, Fair Value | 190,754 | 334,389
Total, Gross Unrealized Loss | 13,769 | 23,203
U.S. government obligations [Member] |  |
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position |  |
Less than Twelve Months, Fair Value | 297,450 | 130,422
Less than 12 Months, Gross Unrealized Loss | 297 | 126
Total, Fair Value | 297,450 | 130,422
Total, Gross Unrealized Loss | 297 | 126
Residential Mortgage-backed Securities [Member] |  |
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position |  |
Less than Twelve Months, Fair Value | 136,572 | 125,826
Less than 12 Months, Gross Unrealized Loss | 7,919 | 14,495
Twelve Months of Longer, Fair Value | 111,500 | 105,705
12 Months or Longer, Gross Unrealized Loss | 32,568 | 43,321
Total, Fair Value | 248,072 | 231,531
Total, Gross Unrealized Loss | 40,487 | 57,816
Collateralized debt obligations [Member] |  |
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position |  |
Less than Twelve Months, Fair Value | 18,279 | 33,037
Less than 12 Months, Gross Unrealized Loss | 197 | 840
Twelve Months of Longer, Fair Value | 9,558 | 12,482
12 Months or Longer, Gross Unrealized Loss | 1,017 | 1,751
Total, Fair Value | 27,837 | 45,519
Total, Gross Unrealized Loss | 1,214 | 2,591
Other asset-backed securities [Member] |  |
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position |  |
Less than Twelve Months, Fair Value | 19,020 | 181,792
Less than 12 Months, Gross Unrealized Loss | 4 | 8,319
Twelve Months of Longer, Fair Value | 195,525 | 204,855
12 Months or Longer, Gross Unrealized Loss | 48,613 | 51,782
Total, Fair Value | 214,545 | 386,647
Total, Gross Unrealized Loss | 48,617 | 60,101
Short-term Investments [Member] |  |
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position |  |
Less than Twelve Months, Fair Value | 6,285 | 5,773
Less than 12 Months, Gross Unrealized Loss | 2 | 1
Total, Fair Value | 6,285 | 5,773
Total, Gross Unrealized Loss | $ 2 | $ 1

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