Investments - Gross Unrealized Losses by Current Severity and Length of Time that Individual Securities Have Been in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 36,190	$ 27,035
Gross Unrealized Losses	(2,026)	(350)
Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(369)	(252)
5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,657)
Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(98)
Three Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	3,059	26,121
Gross Unrealized Losses	(131)	(266)
Three Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(16)	(246)
Three Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(115)
Three Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(20)
Six Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	18,997
Gross Unrealized Losses	(748)
Six Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(254)
Six Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(494)
Six Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses
Nine Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	14,134
Gross Unrealized Losses	(1,147)
Nine Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(99)
Nine Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,048)
Nine Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses
Twelve Months[Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses		702
Gross Unrealized Losses		(6)
Twelve Months[Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(6)
Twelve Months[Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses
Twelve Months[Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses
Greater Than 12 Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses		212
Gross Unrealized Losses		(78)
Greater Than 12 Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses
Greater Than 12 Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses
Greater Than 12 Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		$ (78)