Basis of Presentation, Nature of Business and Significant Accounting Policies and Practices - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2017 USD ($)	Aug. 31, 2017
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Number of reporting Segment | Segment	1
Highly liquid investments with original maturity	3 months
Restricted cash	$ 14,657,000	$ 12,253,000
Interest income on restricted cash deposits	28,969	36,532
Allowance for uncollectible premiums	290,300	0	$ 0
Bad debt expense, uncollectible premiums	290,300	0	0
Ceding commission income	62,450,000	74,157,000	8,600,000
Amounts recoverable under reinsurance program	$ 0	0	0
Operating lease expiration year	2023
Goodwill and intangible assets impairment	$ 0	0	0
Impairment of long-lived assets	0	0	0
Cash deposits in excess of FDIC insurance limits	267,400,000	244,600,000
Deposits held	$ 250,000
Right-of-use assets				$ 2,500,000
Lease liabilities				$ 2,600,000
Convertible Senior Notes [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Aggregate principal amount		75,800,000			$ 136,800,000
Interest rate					5.875%	5.875%
Notes maturity date	Aug. 01, 2037
Convertible Notes outstanding	$ 23,400,000
Building [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Property plant and equipment useful life	40 years
Computer Hardware and Software [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Property plant and equipment useful life	3 years
NBIC [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Ceding commission income	$ 62,400,000	73,000,000
Policy Acquisition Costs [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Ceding commission income	$ 47,000,000	54,900,000	$ 8,600,000
Minimum [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Allowance for credit losses for amounts outstanding, term	90 days
Minimum [Member] | Office and Furniture Equipment [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Property plant and equipment useful life	3 years
Maximum [Member] | Office and Furniture Equipment [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Property plant and equipment useful life	7 years
Regulatory State Deposit [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Restricted cash	$ 5,700,000	3,200,000
Heritage Property and Casualty Insurance Company [Member]
Basis Of Presentation Nature Of Business And Significant Accounting Policies And Practices [Line Items]
Restricted cash	$ 9,000,000	$ 9,000,000