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	Mar. 31, 2015	Dec. 31, 2014
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 21,699	$ 38,072
Gross Unrealized Losses	(277)	(682)
Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(276)	(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	11,880	17,505
Gross Unrealized Losses	(110)	(90)
Less than or equals to three months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(109)	(90)
Less than or equals to three months | Percentage of amortized cost greater than 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1)
Greater than 12 months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	9,819	20,567
Gross Unrealized Losses	(167)	(592)
Greater than 12 months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	$ (167)	$ (592)