Amount and Percentage of Fixed Maturities and Short-term Investments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Rating	AA
Fair Value	$ 1,888,069	$ 1,882,245
Percent of Total	100.00%
Extremely strong
Rating	AAA
Fair Value	291,600
Percent of Total	15.00%
Very strong
Rating	AA
Fair Value	1,048,922
Percent of Total	56.00%
Strong
Rating	A
Fair Value	422,777
Percent of Total	22.00%
Adequate
Rating	BBB
Fair Value	108,299
Percent of Total	6.00%
Speculative
Rating	BB & Below
Fair Value	11,665
Percent of Total	1.00%
Not rated
Rating	NR
Fair Value	$ 4,806
Percent of Total	0.00%

For an unclassified balance sheet, total of debt securities categorized neither as held-to-maturity nor trading.

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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+ Details