Investments - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Other Invested Assets	$ 1,720				$ 1,960				$ 1,720	$ 1,960
Non-credit OTTI on fixed maturities in a net unrealized net gain position									500	400
Gross Unrealized Loss	16,184				30,199				16,184	30,199
OTTI losses									2,064	150	$ 1,698
Net Realized Gains	37,141	$ 5,190	$ 1,694	$ 1,049	4,043	$ 1,586	$ 1,960	$ 1,597	45,073	9,186	8,373
Fixed Maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	14,512				24,414				14,512	24,414
OTTI losses										200
Net Realized Gains									792	1,410	(1,170)
Equity Securities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 1,672				$ 5,785				1,672	5,785
OTTI losses									2,100		1,700
Net Realized Gains									$ 43,262	$ 8,438	$ 9,796
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%				5.00%				5.00%	5.00%
Gross Unrealized Loss	$ 700				$ 1,000				$ 700	$ 1,000
Mortgage-Backed and Asset-Backed Securities
Schedule of Investments [Line Items]
Securities, effective maturities									5 years
Mortgage-Backed and Asset-Backed Securities | Fixed Maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 7,338				$ 9,722				$ 7,338	$ 9,722