Investments - Major Categories of Net Investment Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Net Amount at Risk by Product and Guarantee [Line Items]
Interest from fixed maturities	$ 20,027	$ 20,765	$ 62,019	$ 52,471
Amortization of bond premiums and discounts	(6,208)	(7,710)	(22,634)	(16,717)
Dividends from equities	593	393	1,904	1,097
Interest from cash and cash equivalents and short-term investments	4,525	2,994	11,684	8,743
Interest on other receivables	1,027	871	6,242	4,971
Other income	733	1,378	4,091	2,723
Interest on deposits held with clients	377	317	988	1,013
Investment expenses	(1,416)	(510)	(3,299)	(1,196)
Net investment income	$ 19,658	$ 18,498	$ 60,995	$ 53,105