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Investments - Credit Ratings (Details) (USD $)
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3 Months Ended | ||||||
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Mar. 31, 2014
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Mar. 31, 2013
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Dec. 31, 2013
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| Schedule of Available-for-sale Securities [Line Items] | |||||||
| OTTI | $ 0 | $ 0 | |||||
| Amortized cost | 3,096,675,000 | 3,127,792,000 | |||||
| Fair value | 3,168,819,000 | 3,162,067,000 | |||||
| % of Total fair value | 100.00% | 100.00% | |||||
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U.S. treasury bonds [Member]
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| Schedule of Available-for-sale Securities [Line Items] | |||||||
| Amortized cost | 16,616,000 | [1] | 16,622,000 | [1] | |||
| Fair value | 17,135,000 | [1] | 17,209,000 | [1] | |||
| % of Total fair value | 0.50% | [1] | 0.50% | [1] | |||
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U.S. agency bonds [Member]
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| Schedule of Available-for-sale Securities [Line Items] | |||||||
| Amortized cost | 1,264,253,000 | [1] | 1,299,239,000 | [1] | |||
| Fair value | 1,243,666,000 | [1] | 1,270,763,000 | [1] | |||
| % of Total fair value | 39.20% | [1] | 40.20% | [1] | |||
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AAA [Member]
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| Schedule of Available-for-sale Securities [Line Items] | |||||||
| Amortized cost | 216,123,000 | [1] | 210,872,000 | [1] | |||
| Fair value | 231,871,000 | [1] | 222,417,000 | [1] | |||
| % of Total fair value | 7.30% | [1] | 7.00% | [1] | |||
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AA Plus, AA, AA Minus [Member]
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| Schedule of Available-for-sale Securities [Line Items] | |||||||
| Amortized cost | 138,743,000 | [1] | 236,424,000 | [1] | |||
| Fair value | 147,363,000 | [1] | 242,986,000 | [1] | |||
| % of Total fair value | 4.70% | [1] | 7.70% | [1] | |||
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A Plus, A, A Minus [Member]
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| Schedule of Available-for-sale Securities [Line Items] | |||||||
| Amortized cost | 688,926,000 | [1] | 619,148,000 | [1] | |||
| Fair value | 729,160,000 | [1] | 651,248,000 | [1] | |||
| % of Total fair value | 23.00% | [1] | 20.60% | [1] | |||
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BBB Plus, BBB, BBB Minus [Member]
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| Schedule of Available-for-sale Securities [Line Items] | |||||||
| Amortized cost | 716,179,000 | [1] | 689,532,000 | [1] | |||
| Fair value | 742,241,000 | [1] | 701,529,000 | [1] | |||
| % of Total fair value | 23.50% | [1] | 22.20% | [1] | |||
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BB Plus or Lower [Member]
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| Schedule of Available-for-sale Securities [Line Items] | |||||||
| Amortized cost | 55,835,000 | [1] | 55,955,000 | [1] | |||
| Fair value | $ 57,383,000 | [1] | $ 55,915,000 | [1] | |||
| % of Total fair value | 1.80% | [1] | 1.80% | [1] | |||
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