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Fair Value Measurements - Additional Information (Detail) (USD $)
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Sep. 30, 2012
Security
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Dec. 31, 2011
Security
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|---|---|---|
| Schedule of Available-for-sale Securities [Line Items] | ||
| Percentage of carrying value of investments from external pricing services | 96.00% | |
| Percentage of carrying value of investments from brokers | 1.00% | |
| Percentage of carrying value of investments from internal matrices and calculations | 3.00% | |
| Prices obtained for the number of securities from brokers | 6 | 5 |
| Number of securities on which company internally determined the prices | 18 | 15 |
| Fixed maturity securities available for sale, at fair value | $ 2,878,625,000 | $ 2,682,142,000 |
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Level 3
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| Schedule of Available-for-sale Securities [Line Items] | ||
| Fixed maturity securities available for sale, at fair value | 51,792,000 | 43,759,000 |
| Estimated increase in Basis Spread of fixed maturity securities | 0.10% | |
| Reduction in reported fair value | $ 200,000 |