Reinsurance (Details)				3 Months Ended
	Jan. 01, 2018 USD ($)	Jun. 01, 2017 USD ($) catastrophic_event reinsurer	Jun. 01, 2016 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Rate
Reinsurance Retention Policy [Line Items]
Quota Share Agreement, Cession Rate | Rate						20.00%
Prepaid reinsurance premiums				$ 127,888,000		$ 201,904,000
Cash recoveries under reinsurance agreements				$ 122,512,000	$ 2,484,000
Flood premiums ceded (percentage)				100.00%
Insurance commissions and fees under Flood Program				$ 388,000	$ 294,000
Reinsurance Multi-Year Limit		$ 87,500,000
Reinsurance cost synergies		20,000,000
Minimum
Reinsurance Retention Policy [Line Items]
ISO Catastrophe Definition		25,000,000
Second And Subsequent Catastrophic Events | Maximum
Reinsurance Retention Policy [Line Items]
Amount of risk reinsured in excess of stated retention		2,747,500,000
Unaffiliated Private Reinsurers | Maximum
Reinsurance Retention Policy [Line Items]
Aggregate Coverage for Covered Losses Under Reinsurance Contract		$ 1,928,000,000
Catastrophe | First Catastrophic Event
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Coverage Percentage Hurricane Event | catastrophic_event		0.010
Catastrophe | First Catastrophic Event | Maximum
Reinsurance Retention Policy [Line Items]
Stated amount of upper retention limit per policy		$ 55,000,000
Catastrophe | Second Catastrophic Event
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Coverage Percentage Hurricane Event | catastrophic_event		0.020
Catastrophe | Second And Subsequent Catastrophic Events | Maximum
Reinsurance Retention Policy [Line Items]
Stated amount of upper retention limit per policy		$ 30,000,000
Catastrophe | Florida Hurricane Catastrophe Fund | FLORIDA
Reinsurance Retention Policy [Line Items]
Aggregate Coverage for Covered Losses Under Reinsurance Contract			$ 789,000,000
Reinsurance Retention Policy, Participation Rate			45.00%
Catastrophe | Unaffiliated Private Reinsurers
Reinsurance Retention Policy [Line Items]
Number of private reinsurers | reinsurer		43
CAT Aggregate
Reinsurance Retention Policy [Line Items]
Percentage of Gross Earned Premiums				5.00%
Reinsurance Recoverables on Unpaid Losses, Gross				$ 19,000,000
CAT Aggregate | Maximum
Reinsurance Retention Policy [Line Items]
Stated amount of upper retention limit per policy	$ 20,000,000
BlueLine | Catastrophe | Second And Subsequent Catastrophic Events | Maximum
Reinsurance Retention Policy [Line Items]
Stated amount of upper retention limit per policy		$ 5,000,000