Reinsurance - Additional information (Detail)		1 Months Ended			3 Months Ended		4 Months Ended	5 Months Ended	9 Months Ended		12 Months Ended
	Feb. 29, 2016 USD ($)	May 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Apr. 30, 2015 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($) Reinsurer	Sep. 30, 2018 USD ($)	May 31, 2018	Sep. 30, 2018 USD ($) Reinsurer Layer	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchase of reinsurance from third party					$ 115,926,000	$ 57,855,000			$ 356,748,000	$ 182,189,000
Reinsurance payable					217,022,000		$ 217,022,000		217,022,000		$ 17,577,000
Prepaid reinsurance premiums					282,649,000		282,649,000		282,649,000		227,764,000
Net of prepaid reinsurance premium as attachment point					324,826,000		324,826,000		324,826,000		$ 357,357,000
Percentage of aggregate contract											25.00%
Unpaid losses and loss adjustment expenses					421,095,000	489,580,000	421,095,000		$ 421,095,000	489,580,000	$ 470,083,000	$ 488,610,000	$ 122,785,000	$ 140,137,000
Number of reinstatements available | Reinsurer									2
Aggregate participation, losses and loss adjustment expenses					(3,205,000)	3,599,000			$ 14,660,000	$ 1,473,000
Insurance Claims [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Unpaid losses and loss adjustment expenses					1,000,000		1,000,000		1,000,000
Property Per Risk Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable					27,000,000		27,000,000		27,000,000
Coverage limit									$ 9,000,000
Hartford Steam Boiler [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									100.00%
Heritage P&C [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									20.00%
Primary retention									$ 16,000,000
Primary retention					20,000,000		20,000,000		20,000,000
Primary retention									16,000,000
Agreement of coverage	3 years	3 years
Additional maturity period of collateral notes	2 years	2 years
Reinsurance agreement	3 years	3 years
Heritage P&C [Member] | States Other Than Hawaii [Member] | Notes Due March 2020 [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust		$ 35,000,000
Heritage P&C [Member] | States Other Than Hawaii [Member] | Notes Due February 2019 [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust	$ 250,000,000
Heritage P&C [Member] | States Other Than Hawaii [Member] | Class D Notes Due February 2019 [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust	150,000,000
Heritage P&C [Member] | States Other Than Hawaii [Member] | Class E Notes Due February 2019 [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust	$ 100,000,000
Heritage P&C [Member] | States Other Than Hawaii [Member] | Two Thousand Fifteen Class B And C Notes [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Agreement of coverage				3 years
Heritage P&C [Member] | States Other Than Hawaii [Member] | Class B Notes Due April 2017 [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust				$ 97,500,000
Heritage P&C [Member] | States Other Than Hawaii [Member] | Class C Notes Due April 2017 [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust				30,000,000
Heritage P&C [Member] | Top and Aggregate Layer [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention					15,000,000		15,000,000		15,000,000
Aggregate participation, losses and loss adjustment expenses									5,000,000
Heritage P&C [Member] | Maximum [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Aggregate participation, losses and loss adjustment expenses									36,000,000
Zephyr [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									16,000,000
Primary retention					20,000,000		20,000,000		20,000,000
Primary retention									16,000,000
Zephyr [Member] | Top and Aggregate Layer [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention					15,000,000		15,000,000		15,000,000
Aggregate participation, losses and loss adjustment expenses									5,000,000
Heritage And Zephyr
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention						$ 2,600,000,000
Primary retention					20,000,000		20,000,000		20,000,000
Number of reinstatements available | Reinsurer						2
Heritage And Zephyr | Florida [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention						$ 1,750,000,000
Heritage And Zephyr | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention						$ 731,000,000
2018 - 2019 Reinsurance Program [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance purchase limit					3,400,000,000		3,400,000,000		3,400,000,000
Purchase of reinsurance from third party									3,500,000,000
Reinsurance payable					252,000,000		$ 252,000,000		$ 252,000,000
FHCF [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									45.00%
Gross Quota Share [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of gross quota share							8.00%	18.75%	18.75%
Reinsurance recoveries on paid losses					$ 1,000,000,000		$ 1,000,000,000		$ 1,000,000,000
Net Quota Share [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Net lines quota share occurrence limit									20,000,000
Percentage of ceded net premium and losses											60.00%			8.00%
Percentage of renewed ceded net premium and losses											49.50%
25% Aggregate Coverage [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of aggregate contract											25.00%
Aggregate contract coverage limit									13,500,000
Primary retention									18,500,000
Franchise deductible amount									$ 1,000,000
Aggregate contract expiration date									May 31, 2018
42.5% Aggregate Coverage [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of aggregate contract					42.50%		42.50%		42.50%
Aggregate contract coverage limit							$ 20,000,000
Primary retention							3,000,000
Franchise deductible amount							$ 1,500,000
Aggregate contract expiration date							Dec. 31, 2018
92.00% Aggregate Coverage [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of aggregate contract					92.00%		92.00%		92.00%
Aggregate contract coverage limit							$ 20,000,000
Primary retention							$ 20,000,000
Aggregate contract expiration date							Dec. 31, 2018
Facultative Reinsurance [Member] | Minimum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Facultative reinsurance purchase amount									$ 10,000,000
Facultative Reinsurance [Member] | Maximum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable					$ 10,000,000		$ 10,000,000		10,000,000
General Excess of Loss 2018-2019 Reinsurance Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									$ 300,000
Number of loss applied to reinsurance contract | Reinsurer									1
Number of layers in excess of rentention loss | Layer									2
General Excess of Loss 2018-2019 Reinsurance Program [Member] | NBIC [Member] | First Layer Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									$ 450,000
Primary retention									300,000
General Excess of Loss 2018-2019 Reinsurance Program [Member] | NBIC [Member] | Second Layer Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									2,750,000
Primary retention									750,000
General Excess of Loss 2018-2019 Reinsurance Program [Member] | NBIC [Member] | Casualty Second Layer [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									1,250,000
Primary retention									$ 750,000
General Excess of Loss 2018-2019 Reinsurance Program [Member] | NBIC [Member] | First and Second Layer Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of gross quota share									92.00%
Additional coverage percentage of gross quota share									8.00%
Facultative 2018 - 2019 Reinsurance Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance loss limit									$ 3,750,000
Facultative 2018 - 2019 Reinsurance Program [Member] | NBIC [Member] | Minimum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable					3,500,000		3,500,000		3,500,000
FHCF Layer [Member] | Heritage P&C [Member] | Property Per Risk Coverage [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									$ 414,000,000
Percentage of maximum provisional limit									45.00%
Estimated provisional limit percentage calculation base amount									$ 1,300,000,000
FHCF Layer [Member] | Heritage And Zephyr | Property Per Risk Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									45.00%
Shared Layers [Member] | Property Per Risk Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchase of reinsurance from third party									$ 372,000,000
Shared coverage amount available subject to seasonal aggregate									20,000,000
Seasonal aggregate amount									68,000,000
Shared Layers [Member] | Property Per Risk Coverage [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchase of reinsurance from third party									372,000,000
Shared coverage amount available subject to seasonal aggregate									20,000,000
Seasonal aggregate amount									68,000,000
Shared Layers [Member] | Heritage And Zephyr | Property Per Risk Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									352,000,000
Shared Layers [Member] | Heritage And Zephyr | Property Per Risk Coverage [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									352,000,000
Layers Above FHCF -Florida Program [Member] | Heritage P&C [Member] | States Other Than Hawaii [Member] | 2017-1 Notes [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust			$ 125,000,000
Agreement of coverage			3 years
Additional maturity period of collateral notes			2 years
Reinsurance agreement			3 years
Layers Above FHCF -Florida Program [Member] | Heritage P&C [Member] | States Other Than Hawaii [Member] | Class A Notes Due April Two Thousand Seventeen [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust				$ 150,000,000
Agreement of coverage				3 years
Additional maturity period of collateral notes				2 years
Reinsurance agreement				3 years
Top Hawaii Only Layer [Member] | Property Per Risk Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchase of reinsurance from third party									$ 26,000,000
2017 - 2018 Reinsurance Program [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Number of reinstatements available | Reinsurer									2
2017 - 2018 Reinsurance Program [Member] | Insurance Claims [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Unpaid losses and loss adjustment expenses					1,000,000		1,000,000		$ 1,000,000
2017 - 2018 Reinsurance Program [Member] | Property Per Risk Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable					$ 27,000,000		$ 27,000,000		27,000,000
Coverage limit									$ 9,000,000
2017 - 2018 Reinsurance Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of aggregate contract					25.00%		25.00%		25.00%
Aggregate contract coverage limit									$ 13,500,000
Primary retention									18,500,000
Franchise deductible amount									$ 1,000,000
Aggregate contract expiration date									May 31, 2018
2017 - 2018 Reinsurance Program [Member] | Citrus Re Ltd [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									26.00%
Purchase of reinsurance from third party									$ 687,500,000
Coverage arrangements, period									multi-year
2017 - 2018 Reinsurance Program [Member] | Citrus Re Ltd [Member] | Class A and B and C Due in May 2018 [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchase of reinsurance from third party									$ 277,500,000
Notes maturity date									May 31, 2018
2017 - 2018 Reinsurance Program [Member] | Citrus Re Ltd [Member] | Class D and E Due in Two-year Period [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchase of reinsurance from third party									$ 250,000,000
Additional maturity period of collateral notes									2 years
2017 - 2018 Reinsurance Program [Member] | Citrus Re Ltd [Member] | Notes Due in Three-year Period [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchase of reinsurance from third party									$ 160,000,000
Notes maturity period									3 years
2017-2018 NBIC Program [Member] | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable Including Reinsurance Premium Paid [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Concentration risk percentage									10.00%
2017-2018 NBIC Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									$ 1,000,000,000
General Excess of Loss [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									$ 300,000
Number of loss applied to reinsurance contract | Reinsurer									1
Number of layers in excess of rentention loss | Layer									2
General Excess of Loss [Member] | NBIC [Member] | First Layer Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									$ 450,000
Primary retention									300,000
General Excess of Loss [Member] | NBIC [Member] | Second Layer Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									2,750,000
Primary retention									750,000
General Excess of Loss [Member] | NBIC [Member] | Casualty Second Layer [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									1,250,000
Primary retention									$ 750,000
General Excess of Loss [Member] | NBIC [Member] | First and Second Layer Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of gross quota share									81.25%
Additional coverage percentage of gross quota share									18.75%
Facultative 2017 - 2018 Reinsurance Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance loss limit									$ 2,500,000
Facultative 2017 - 2018 Reinsurance Program [Member] | NBIC [Member] | Minimum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable					$ 3,500,000		$ 3,500,000		$ 3,500,000
Umbrella Facultative Reinsurance | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of quota share									90.00%
Additional percentage of quota share									100.00%
Umbrella Facultative Reinsurance | NBIC [Member] | Maximum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Limits of liability					1,000,000		1,000,000		$ 1,000,000
Additional limits of liability					4,000,000		4,000,000		4,000,000
Catastrophe [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance purchase limit					632,000,000		632,000,000		632,000,000
Catastrophe [Member] | Citrus [Member] | Property Per Risk Coverage [Member] | States Other Than Hawaii [Member] | Two Thousand Fifteen C And C Series Bond And Two Thousand Sixteen D and E Bond And Two Thousand Seventeen Two Bond [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Collateralized by a reinsurance trust					412,500,000		412,500,000		412,500,000
Additional coverage of second catastrophe reinsurance agreement									$ 5,000,000
Catastrophe [Member] | 2018 - 2019 Reinsurance Program [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance premium, amortization period									12 months
Catastrophe [Member] | Aggregate Coverage | Property Per Risk Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									$ 700,000,000
Prepaid reinsurance premiums					632,000,000		632,000,000		632,000,000
Catastrophe [Member] | Aggregate Coverage | Heritage P&C [Member] | Property Per Risk Coverage [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									984,000,000
Prepaid reinsurance premiums					606,000,000		606,000,000		606,000,000
Net of prepaid reinsurance premium as attachment point					40,000,000		40,000,000		40,000,000
Catastrophe [Member] | Aggregate Coverage | Heritage And Zephyr
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									1,100,000,000
Prepaid reinsurance premiums					669,000,000		669,000,000		669,000,000
Net of prepaid reinsurance premium as attachment point					40,000,000		40,000,000		40,000,000
Catastrophe [Member] | Multi-Zonal Layers [Member] | Heritage P&C [Member] | Property Per Risk Coverage [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Coverage of first catastrophe reinsurance agreement									254,000,000
Catastrophe [Member] | Multi-Zonal Layers [Member] | Zephyr [Member] | Property Per Risk Coverage [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Prepaid reinsurance premiums					254,000,000		254,000,000		254,000,000
Coverage of first catastrophe reinsurance agreement									302,000,000
Catastrophe [Member] | Multi-Zonal Layers [Member] | Zephyr [Member] | Property Per Risk Coverage [Member] | Florida [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									254,000,000
Catastrophe [Member] | Multi-Zonal Layers [Member] | Zephyr [Member] | Property Per Risk Coverage [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									254,000,000
Catastrophe [Member] | A Top And Aggregate Multi Zonal Layer [Member] | Zephyr [Member] | Property Per Risk Coverage [Member] | Florida [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									48,000,000
Catastrophe [Member] | A Top And Aggregate Multi Zonal Layer [Member] | Zephyr [Member] | Property Per Risk Coverage [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchased reinstatement premium									48,000,000
Catastrophe [Member] | 2017 - 2018 Reinsurance Program [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Purchase of reinsurance from third party									$ 2,600,000,000
Description of hurricane losses coverage and protection									Hurricane losses in states other than Hawaii were covered under the Heritage P&C program with the exception of the FHCF coverage and the series 2015, 2016 and 2017 catastrophe bonds which cover events solely occurring in Florida. Management deemed this reinsurance protection to be sufficient given the level of catastrophe exposure in 2017 for Alabama, Georgia, North Carolina and South Carolina.
Catastrophe [Member] | 2017 - 2018 Reinsurance Program [Member] | Property Per Risk Coverage [Member] | Florida [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Unpaid losses and loss adjustment expenses					1,750,000,000		1,750,000,000		$ 1,750,000,000
Catastrophe [Member] | 2017 - 2018 Reinsurance Program [Member] | Property Per Risk Coverage [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Unpaid losses and loss adjustment expenses					731,000,000		731,000,000		731,000,000
Catastrophe [Member] | 2017-2018 NBIC Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention					1,300,000		1,300,000		$ 1,300,000
Quota Share [Member] | 2018 - 2019 Reinsurance Program [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance premium, amortization period									12 months
First Catastrophic Event [Member] | Heritage P&C [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									20.00%
Primary retention					878,000,000		878,000,000		$ 878,000,000
First Catastrophic Event [Member] | Heritage P&C [Member] | Maximum [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Aggregate participation, losses and loss adjustment expenses									$ 727,000
First Catastrophic Event [Member] | Zephyr [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									3.80%
Primary retention					386,000,000		386,000,000		$ 386,000,000
First Catastrophic Event [Member] | Zephyr [Member] | Maximum [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Aggregate participation, losses and loss adjustment expenses									12,000,000
First Catastrophic Event [Member] | 2018 - 2019 Reinsurance Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									1,000,000,000
Primary retention					12,800,000		12,800,000		12,800,000
First Catastrophic Event [Member] | 2018 - 2019 Reinsurance Program [Member] | Heritage P&C [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									1,600,000,000
Primary retention					20,000,000		20,000,000		20,000,000
First Catastrophic Event [Member] | 2018 - 2019 Reinsurance Program [Member] | Zephyr [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									801,000,000
Primary retention					20,000,000		20,000,000		20,000,000
First Catastrophic Event [Member] | 2018 - 2019 Reinsurance Program [Member] | Osprey
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention					4,000,000		4,000,000		$ 4,000,000
Second Catastrophic Event [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Number reinsurance reinstatement provisions | Reinsurer									1
Percentage of reinsurance reinstatement provisions premium									100.00%
Second Catastrophic Event [Member] | Heritage P&C [Member] | States Other Than Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									11.50%
Primary retention					420,000,000		420,000,000		$ 420,000,000
Second Catastrophic Event [Member] | Zephyr [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage comprising aggregate participation									117.70%
Primary retention					386,000,000		386,000,000		$ 386,000,000
Second Catastrophic Event [Member] | Zephyr [Member] | Maximum [Member] | Hawaii [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Aggregate participation, losses and loss adjustment expenses									56,000,000
Second Catastrophic Event [Member] | 2018 - 2019 Reinsurance Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									8,800,000
Second Catastrophic Event [Member] | 2018 - 2019 Reinsurance Program [Member] | Heritage P&C [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									16,000,000
Second Catastrophic Event [Member] | 2018 - 2019 Reinsurance Program [Member] | Zephyr [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Primary retention									$ 16,000,000
Second Catastrophic Event [Member] | Gross Quota Share [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Percentage of gross quota share									81.25%
Percentage of remaining gross quota share									18.75%
Second Catastrophic Event [Member] | Net Quota Share [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reduction in reinsurance reinstatement net retention amount									$ 20,000,000
Aggregate Excess For All Catastrophe Losses Excluding Named Tropical Storms
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable											$ 20,000,000
Aggregate Excess For All Catastrophe Losses Excluding Named Tropical Storms | Minimum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Limit of aggregate losses under aggregate contract											21,500,000
Aggregate Excess For All Catastrophe Losses Excluding Named Tropical Storms | 2017 - 2018 Reinsurance Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable					20,000,000		20,000,000		20,000,000
Aggregate Excess For All Catastrophe Losses Excluding Named Tropical Storms | 2017 - 2018 Reinsurance Program [Member] | NBIC [Member] | Minimum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Limit of aggregate losses under aggregate contract									21,500,000
Named Tropical Storm Losses
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable											12,000,000
Named Tropical Storm Losses | Minimum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Limit of aggregate losses under aggregate contract											$ 8,000,000
Named Tropical Storm Losses | 2017 - 2018 Reinsurance Program [Member] | NBIC [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance payable					$ 12,000,000		$ 12,000,000		12,000,000
Named Tropical Storm Losses | 2017 - 2018 Reinsurance Program [Member] | NBIC [Member] | Minimum [Member]
Reinsurance Premiums For Insurance Companies By Product Segment [Line Items]
Reinsurance loss limit									$ 8,000,000