Summary of Activity Related to Other-Than-Temporary Impairment Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended							12 Months Ended
	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 Investment	Dec. 31, 2011 Investment	Dec. 31, 2010 Investment
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Number of Securities								4	22	20
Total other-than-temporary impairment losses	$ 208	$ 496	$ 198	$ 1,280	$ 1,241	$ 833	$ 263	$ 902	$ 3,617	$ 2,222
Portion of loss recognized in other comprehensive income (before tax)			44	691	618	301	22	44	1,632	1,142
Net other-than-temporary impairment losses recognized in earnings	208	496	154	589	623	532	241	858	1,985	1,080
Corporate And Other Bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Number of Securities									1
Total other-than-temporary impairment losses									109
Net other-than-temporary impairment losses recognized in earnings									109
Residential Mortgage Backed Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Number of Securities								1	19	18
Total other-than-temporary impairment losses								55	2,616	1,835
Portion of loss recognized in other comprehensive income (before tax)								44	1,632	1,142
Net other-than-temporary impairment losses recognized in earnings								11	984	693
Equity Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Number of Securities								3	2	2
Total other-than-temporary impairment losses								847	892	387
Net other-than-temporary impairment losses recognized in earnings								$ 847	$ 892	$ 387