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	Dec. 31, 2016	Dec. 31, 2015
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	$ 86,000	$ 57,573
Gross Unrealized Losses	(3,079)	(1,432)
Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,693)	(769)
Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,386)	(662)
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	58,550	20,899
Gross Unrealized Losses	(1,886)	(130)
Less than or equals to three months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(1,461)	(130)
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	27,193	7,036
Gross Unrealized Losses	(1,186)	(465)
Less than or equals to six months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(232)
Less than or equals to six months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses	(954)	(465)
Less than or equals to nine months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	152	14,057
Gross Unrealized Losses		(395)
Less than or equals to nine months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(197)
Less than or equals to nine months | Percentage of amortized cost 5% to 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(197)
Less than or equals to nine months | Percentage of amortized cost greater than 10%
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(1)
Less than or equals to twelve months
Investment Maturity Date [Line Items]
Fair Value of Securities with Gross Unrealized Losses	105	7,892
Gross Unrealized Losses	(7)	(201)
Less than or equals to twelve months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses		(201)
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		7,689
Gross Unrealized Losses		(241)
Greater than 12 months | Percentage of amortized cost less than 5%
Investment Maturity Date [Line Items]
Gross Unrealized Losses		$ (241)