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	Jun. 30, 2013	Dec. 31, 2012
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Loss	$ 37,115	$ 27,035
Gross Unrealized Losses	(1,881)	(350)
Less Than 5% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses	(511)	(252)
5% to 10% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses	(1,371)
Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses		(98)
Three Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Loss	21,915	26,121
Gross Unrealized Losses	(804)	(266)
Three Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses	(386)	(246)
Three Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses	(419)
Three Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses		(20)
Six Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Loss	3,368
Gross Unrealized Losses	(278)
Six Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses	(278)
Nine Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Loss	10,885
Gross Unrealized Losses	(782)
Nine Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses	(108)
Nine Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses	(674)
Twelve Months[Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Loss		702
Gross Unrealized Losses		(6)
Twelve Months[Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses		(6)
Greater Than 12 Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Loss	947	212
Gross Unrealized Losses	(17)	(78)
Greater Than 12 Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses	(17)
Greater Than 12 Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Severity of Gross Unrealized Losses		$ (78)