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	Sep. 30, 2015	Dec. 31, 2014
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 48,425	$ 38,072
Gross Unrealized Losses	(963)	(682)
Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(838)	(682)
Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(124)
Percentage of amortized cost greater than 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1)
Less than or equals to three months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	14,736	17,505
Gross Unrealized Losses	(180)	(90)
Less than or equals to three months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(180)	(90)
Less than or equals to six months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	18,036
Gross Unrealized Losses	(379)
Less than or equals to six months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(254)
Less than or equals to six months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(124)
Less than or equals to six months | Percentage of amortized cost greater than 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1)
Less than or equals to nine months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	7,857
Gross Unrealized Losses	(239)
Less than or equals to nine months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(239)
Greater than 12 months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	7,796	20,567
Gross Unrealized Losses	(165)	(592)
Greater than 12 months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	$ (165)	$ (592)