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	Jun. 30, 2015	Dec. 31, 2014
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 54,261	$ 38,072
Gross Unrealized Losses	(1,081)	(682)
Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,080)	(682)
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	38,745	17,505
Gross Unrealized Losses	(507)	(90)
Less than or equals to three months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(506)	(90)
Less than or equals to three months | Percentage of amortized cost greater than 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1)
Less than or equals to six months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	7,755
Gross Unrealized Losses	(344)
Less than or equals to six months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(344)
Greater than 12 months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	7,761	20,567
Gross Unrealized Losses	(230)	(592)
Greater than 12 months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	$ (230)	$ (592)