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Investments - Credit Ratings (Details) (USD $)
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3 Months Ended | 9 Months Ended | ||||||||
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Sep. 30, 2013
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Sep. 30, 2012
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Sep. 30, 2013
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Sep. 30, 2012
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Dec. 31, 2012
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| Schedule of Available-for-sale Securities [Line Items] | ||||||||||
| OTTI | $ 0 | $ 0 | $ 0 | $ 0 | ||||||
| Amortized cost | 2,620,107,000 | 2,620,107,000 | 2,475,202,000 | |||||||
| Fair value | 2,674,305,000 | 2,674,305,000 | 2,618,697,000 | |||||||
| % of Total fair value | 100.00% | 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 | 16,629,000 | [1] | 16,629,000 | [1] | 42,671,000 | [1] | ||||
| Fair value | 17,374,000 | [1] | 17,374,000 | [1] | 43,931,000 | [1] | ||||
| % of Total fair value | 0.70% | [1] | 0.70% | [1] | 1.70% | [1] | ||||
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U.S. agency bonds [Member]
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| Schedule of Available-for-sale Securities [Line Items] | ||||||||||
| Amortized cost | 1,146,065,000 | [1] | 1,146,065,000 | [1] | 974,331,000 | [1] | ||||
| Fair value | 1,139,011,000 | [1] | 1,139,011,000 | [1] | 1,005,263,000 | [1] | ||||
| % of Total fair value | 42.60% | [1] | 42.60% | [1] | 38.40% | [1] | ||||
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AAA [Member]
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| Schedule of Available-for-sale Securities [Line Items] | ||||||||||
| Amortized cost | 153,544,000 | [1] | 153,544,000 | [1] | 171,136,000 | [1] | ||||
| Fair value | 165,945,000 | [1] | 165,945,000 | [1] | 183,950,000 | [1] | ||||
| % of Total fair value | 6.20% | [1] | 6.20% | [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 | 114,209,000 | [1] | 114,209,000 | [1] | 186,495,000 | [1] | ||||
| Fair value | 121,167,000 | [1] | 121,167,000 | [1] | 196,797,000 | [1] | ||||
| % of Total fair value | 4.50% | [1] | 4.50% | [1] | 7.50% | [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 | 487,189,000 | [1] | 487,189,000 | [1] | 477,236,000 | [1] | ||||
| Fair value | 517,408,000 | [1] | 517,408,000 | [1] | 515,383,000 | [1] | ||||
| % of Total fair value | 19.30% | [1] | 19.30% | [1] | 19.70% | [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 | 646,396,000 | [1] | 646,396,000 | [1] | 587,858,000 | [1] | ||||
| Fair value | 658,567,000 | [1] | 658,567,000 | [1] | 637,089,000 | [1] | ||||
| % of Total fair value | 24.60% | [1] | 24.60% | [1] | 24.30% | [1] | ||||
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BB Plus or Lower [Member]
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| Schedule of Available-for-sale Securities [Line Items] | ||||||||||
| Amortized cost | 56,075,000 | [1] | 56,075,000 | [1] | 35,475,000 | [1] | ||||
| Fair value | $ 54,833,000 | [1] | $ 54,833,000 | [1] | $ 36,284,000 | [1] | ||||
| % of Total fair value | 2.10% | [1] | 2.10% | [1] | 1.40% | [1] | ||||
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