Investments - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) Security	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Security
Schedule of Investments [Line Items]
Other Invested Assets	$ 1,751		$ 1,751		$ 1,960
Non-credit OTTI on fixed maturities in a net unrealized gain position			500		400
Gross Unrealized Loss	$ 11,590		$ 11,590		$ 30,199
OTTI losses		$ 0
Equity Securities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security	35		35		75
Gross Unrealized Loss	$ 1,460		$ 1,460		$ 5,785
OTTI losses	$ 1,000		$ 2,100
Fixed Maturities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security	317		317		413
Gross Unrealized Loss	$ 10,130		$ 10,130		$ 24,414
OTTI losses				$ 200
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%		5.00%		5.00%
Gross Unrealized Loss	$ 800		$ 800		$ 1,000
Mortgage-Backed and Asset-Backed Securities
Schedule of Investments [Line Items]
Securities, effective maturities			4 years 10 months 25 days
Mortgage-Backed and Asset-Backed Securities | Fixed Maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 6,059		$ 6,059		$ 9,722