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, 2017	Dec. 31, 2016
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 74,702	$ 86,000
Gross Unrealized Losses	(2,039)	(3,079)
Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,868)	(1,693)
Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(171)	(1,386)
Less than or equals to three months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	12,453	58,550
Gross Unrealized Losses	(39)	(1,886)
Less than or equals to three months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(39)	(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	299	27,193
Gross Unrealized Losses	(3)	(1,186)
Less than or equals to six months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(3)	(232)
Less than or equals to six months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(954)
Less than or equals to nine months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	34,834	152
Gross Unrealized Losses	(983)
Less than or equals to nine months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(983)
Less than or equals to twelve months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	27,008	105
Gross Unrealized Losses	(1,010)	(7)
Less than or equals to twelve months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(839)
Less than or equals to twelve months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(171)	$ (7)
Greater than 12 months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	108
Gross Unrealized Losses	(4)
Greater than 12 months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	$ (4)