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	Mar. 31, 2017	Dec. 31, 2016
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 85,959	$ 86,000
Gross Unrealized Losses	(2,781)	(3,079)
Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,644)	(1,693)
Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,137)	(1,386)
Less than or equals to three months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	5,394	58,550
Gross Unrealized Losses	(7)	(1,886)
Less than or equals to three months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(7)	(1,461)
Less than or equals to three months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(425)
Less than or equals to six months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	55,253	27,193
Gross Unrealized Losses	(1,708)	(1,186)
Less than or equals to six months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,427)	(232)
Less than or equals to six months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(281)	(954)
Less than or equals to nine months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	25,206	152
Gross Unrealized Losses	(1,060)
Less than or equals to nine months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(204)
Less than or equals to nine months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(856)
Less than or equals to twelve months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses		105
Gross Unrealized Losses		(7)
Less than or equals to twelve months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses		$ (7)
Greater than 12 months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	106
Gross Unrealized Losses	(6)
Greater than 12 months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	$ (6)