Debt (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Margin borrowing facility	Jul. 19, 2013 6.22% interest rate, guaranteed senior notes, due July 20, 2013 to 2015	Dec. 31, 2012 6.22% interest rate, guaranteed senior notes, due July 20, 2013 to 2015	Sep. 30, 2013 Three-month LIBOR plus 4.05% junior subordinated debentures due September 2033	Dec. 31, 2012 Three-month LIBOR plus 4.05% junior subordinated debentures due September 2033	Sep. 30, 2013 Three-month LIBOR plus 3.85% junior subordinated debentures due October 2033	Dec. 31, 2012 Three-month LIBOR plus 3.85% junior subordinated debentures due October 2033
Debt Instrument [Line Items]
Debt	$ 65,009		$ 65,009	$ 60,000		$ 10,000
Debt	20,619	84,929			54,000		10,310	20,619	20,619
Total debt	$ 85,628	$ 84,929