Unpaid Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended
	Sep. 28, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses		$ 91,046,000	$ 82,756,000	$ 91,046,000	$ 82,756,000	$ 83,662,000	$ 87,896,000	$ 66,917,000	$ 54,651,000
Deferred gain on ADC		4,933,000	0	4,933,000	0	$ 2,412,000	$ 0	$ 0	$ 0
Prior-year adverse (favorable) reserve development		(3,401,000)	(2,070,000)	(4,871,000)	(9,426,000)
Offset amount by favorable development		1,500,000		2,400,000
Liability line of business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior-year adverse (favorable) reserve development		(4,200,000)	(865,000)	(5,300,000)	(4,200,000)
Personal automobile line
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior-year adverse (favorable) reserve development		(612,000)		(1,900,000)
Florida homeowners line
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior-year adverse (favorable) reserve development			(697,000)		(2,400,000)
Commercial property line
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior-year adverse (favorable) reserve development			(350,000)		(2,200,000)
Commercial automobile line
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior-year adverse (favorable) reserve development			(604,000)		(932,000)
Other line
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior-year adverse (favorable) reserve development			(446,000)		(277,000)
Adverse development cover reinsurance agreement (ADC)
Liability for Claims and Claims Adjustment Expense [Line Items]
ADC loss development cover, maximum amount	$ 17,500,000.0	4,100,000		7,300,000
Reinsurance retention policy, co-participation (percentage)	10.00%
Increase in reinsurance recoverable		14,500,000		14,500,000
Reinsurance recoverable		3,000,000		3,000,000
Hurricane
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior-year adverse (favorable) reserve development		$ (83,000)		$ (471,000)
Minimum | Adverse development cover reinsurance agreement (ADC)
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance retention policy carried reserves threshold	$ 1,400,000
Unpaid losses and loss adjustment expenses			36,600,000		36,600,000
Maximum | Adverse development cover reinsurance agreement (ADC)
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance retention policy carried reserves threshold	$ 19,500,000
Unpaid losses and loss adjustment expenses			$ 57,500,000		$ 57,500,000