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)