Investments - Additional Information (Detail) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Security	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) Security
Schedule of Investments [Line Items]
Non-credit OTTI on fixed maturities in a net unrealized net gain position		$ 400		$ 500
Gross Unrealized Loss		8,954		$ 29,398
OTTI losses	$ 1,275	$ 150	$ 1,698
Equity Securities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security		42		57
Gross Unrealized Loss		$ 1,767		$ 3,013
Fixed maturities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security		177		368
Gross Unrealized Loss		$ 7,187		$ 26,385
OTTI losses		200
Equity Portfolio
Schedule of Investments [Line Items]
OTTI losses	$ 1,300		$ 1,700
Non-government-backed securities
Schedule of Investments [Line Items]
Gross Unrealized Loss		$ 400		$ 2,600
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment		5.00%		5.00%
Mortgage-backed and asset-backed securities
Schedule of Investments [Line Items]
Securities, effective maturities		4 years 1 month 6 days
Mortgage-backed and asset-backed securities | Fixed maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss		$ 1,083		$ 5,652