Credit Loss Impairments on Fixed Maturity Securities, Other-Than-Temporary Impairment Loss Recognized in Other Comprehensive Income, and Corresponding Changes (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Balance at beginning of period	$ 2,229	$ 2,014
Credit loss impairment recognized in the current period on securities not previously impaired		27
Additional credit loss impairments recognized in the current period on securities previously impaired through other comprehensive income	251	1,004
Accretion of credit loss impairments previously recognized	(835)	(816)
Balance at end of period	$ 1,645	$ 2,229