Amount and Percentage of Fixed Maturities by Rating (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Investment Securities [Line Items]
Rating	AA
Fair Value	$ 2,121,833	$ 1,888,069
Percent of Total	100.00%
Extremely strong
Investment Securities [Line Items]
Rating	AAA
Fair Value	316,511
Percent of Total	15.00%
Very strong
Investment Securities [Line Items]
Rating	AA
Fair Value	1,257,209
Percent of Total	59.00%
Strong
Investment Securities [Line Items]
Rating	A
Fair Value	369,620
Percent of Total	17.00%
Adequate
Investment Securities [Line Items]
Rating	BBB
Fair Value	159,943
Percent of Total	8.00%
Speculative
Investment Securities [Line Items]
Rating	BB & Below
Fair Value	14,780
Percent of Total	1.00%
Not rated
Investment Securities [Line Items]
Rating	NR
Fair Value	$ 3,770
Percent of Total	0.00%

Amount of debt securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

+ Details