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, 2014	Dec. 31, 2013
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 33,663	$ 37,921
Gross Unrealized Losses	(1,015)	(2,291)
Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(559)	(278)
5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(456)	(2,013)
Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	0	0
Three Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	1,198	6,417
Gross Unrealized Losses	(1)	(40)
Three Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1)	(40)
Three Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	0	0
Six Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	3,038	1,653
Gross Unrealized Losses	(16)	(129)
Six Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(16)
Six Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(129)
Six Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	0	0
Nine Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	2,876	15,871
Gross Unrealized Losses	(16)	(852)
Nine Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(16)	(153)
Nine Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(699)
Nine Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	0	0
Twelve Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	996
Gross Unrealized Losses	(8)
Twelve Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(8)
Twelve Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	0	0
Greater Than 12 Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	25,555	13,980
Gross Unrealized Losses	(974)	(1,270)
Greater Than 12 Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(518)	(85)
Greater Than 12 Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(456)	(1,185)
Greater Than 12 Months [Member] | Greater Than 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	$ 0	$ 0