Investments - Additional Information (Detail)		9 Months Ended
		Sep. 30, 2018 USD ($) Entity	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Investments in a single issuer as a percentage of shareholders' equity		5.00%	5.00%
Gross unrealized losses			$ 9,414,000
Gross unrealized losses for 12 months or greater	[1]		1,843,000
Insurance enhanced collateralized mortgage obligations, commercial mortgage-backed, and credit securities		$ 34,200,000
Investments in insurance enhanced municipal bonds		708,000
Ratings without insurance		0
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds		$ 33,500,000
Asset backed and taxable municipal bonds as a percentage of total cash and invested assets		2.20%
Variable Interest Entity, Not Primary Beneficiary
Debt Securities, Available-for-sale [Line Items]
Number of VIE’s | Entity		3
Ownership interest exceeds respective investments		3.00%
New limited partnership that invests in distressed securities and assets and considered a VIE
Debt Securities, Available-for-sale [Line Items]
Significant variable interest in fair value of the non-consolidated VIE		$ 32,100,000	17,800,000
Variable interest entities, maximum exposure to loss		52,600,000	51,300,000
One of the Company's variable interest VIE's, invests in distressed securities and assets
Debt Securities, Available-for-sale [Line Items]
Significant variable interest in fair value of the non-consolidated VIE		19,000,000	26,300,000
Variable interest entities, maximum exposure to loss		33,200,000	40,500,000
Second VIE that provides financing for middle market companies
Debt Securities, Available-for-sale [Line Items]
Significant variable interest in fair value of the non-consolidated VIE		34,200,000	33,800,000
Variable interest entities, maximum exposure to loss		39,400,000	$ 43,800,000
Municipal Bond Insurance Association
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds		6,000,000
Assured Guaranty Corporation
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds		14,500,000
Taxable Municipal Bonds
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds		20,500,000
Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds		12,800,000
Federal Deposit Insurance Corporation
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds		200,000
Pre-Refunded Securities
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds		0
AA Rating
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds		$ 710,000
AA Rating | Maximum
Debt Securities, Available-for-sale [Line Items]
Insurance enhanced municipal bonds as a percentage of total cash and invested assets		0.10%
U.S. Treasury and Agency Obligations
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses		$ 2,051,000
U.S. Treasury and Agency Obligations | AA+ Rating
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses for 12 months or greater		1,772,000
Obligations of States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses		998,000
Obligations of States and Political Subdivisions | A- Rating
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses for 12 months or greater		220,000
Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses		3,878,000
Mortgage Backed Securities | AA+ Rating
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses for 12 months or greater		$ 1,315,000
Percentage of unrealized losses for 12 months or greater		99.30%
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses		$ 1,641,000
Weighted average credit enhancement		23.20%
Asset-backed Securities | A Rating
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses for 12 months or greater		$ 394,000
Percentage of unrealized losses for 12 months or greater		78.20%
Commercial Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses		$ 5,876,000
Weighted average credit enhancement		49.10%
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds		$ 12,800,000
Commercial Mortgage-Backed Securities | A- Rating
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses for 12 months or greater		2,627,000
Foreign Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses		2,814,000
Foreign Corporate Bonds | Investment Grade Rating
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses for 12 months or greater		$ 590,000
Percentage of unrealized losses for 12 months or greater		95.50%
Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses		$ 9,601,000
Corporate Bonds | Investment Grade Rating
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses for 12 months or greater		$ 976,000
Percentage of unrealized losses for 12 months or greater		89.00%
Collateralized Mortgage Obligations
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds		$ 200,000

[1] Fixed maturities in a gross unrealized loss position for twelve months or longer are primarily comprised of non-credit losses on investment grade securities where management does not intend to sell, and it is more likely than not that the Company will not be forced to sell the security before recovery. The Company has analyzed these securities and has determined that they are not other than temporarily impaired.