Investments - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) Security	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Security	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) Security
Schedule of Investments [Line Items]
Non-credit OTTI on fixed maturities in an unrealized gain position			$ 600		$ 700
Gross Unrealized Loss	$ 23,988		23,988		$ 11,651
OTTI losses	1,275	$ 0	1,698	$ 0
Credit loss portion of the OTTI losses on fixed maturities	$ 2,400	2,700	$ 2,400	2,700
Bond sold for credit loss		$ 1,500		$ 1,500
Equity Securities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security	58		58		15
Gross Unrealized Loss	$ 7,318		$ 7,318		$ 1,304
Fixed maturities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security	252		252		259
Gross Unrealized Loss	$ 16,670		$ 16,670		$ 10,347
Maximum | Equity Securities
Schedule of Investments [Line Items]
Percentage of fair value under carrying value	80.00%		80.00%
Non-government-backed securities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 700		$ 700		$ 500
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%		5.00%		5.00%
Mortgage-backed and asset-backed securities
Schedule of Investments [Line Items]
Securities, effective maturities			4 years 7 months 6 days
Mortgage-backed and asset-backed securities | Fixed maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 2,820		$ 2,820		$ 2,198