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	Dec. 31, 2014	Dec. 31, 2013
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 38,072	$ 37,921
Gross Unrealized Losses	(682)	(2,291)
Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(682)	(278)
5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(2,013)
Three Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	17,505	6,417
Gross Unrealized Losses	(90)	(40)
Three Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(90)	(40)
Six Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses		1,653
Gross Unrealized Losses		(129)
Six Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(129)
Nine Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses		15,871
Gross Unrealized Losses		(852)
Nine Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(153)
Nine Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(699)
Greater Than 12 Months [Member]
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	20,567	13,980
Gross Unrealized Losses	(592)	(1,270)
Greater Than 12 Months [Member] | Less Than 5% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(592)	(85)
Greater Than 12 Months [Member] | 5% to 10% [Member]
Investment Maturity Date [Line Items]
Gross Unrealized Losses		$ (1,185)