Reinsurance - Narrative (Details) $ in Thousands			12 Months Ended
	Jun. 01, 2017 USD ($) reinsurer	Jun. 01, 2016 USD ($)	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 01, 2016 Rate
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid Reinsurance Premiums			$ 201,904	$ 132,564
Policyholder Benefits and Claims Incurred, Assumed and Ceded			$ 186,104	18,412
Percentage of Flood Premiums Ceded			100.00%
Insurance Commissions and Fees, Flood Program			$ 1,255	$ 1,056	$ 959
Increase (Decrease) in Ceded Losses and LAE Incurred			$ (518,293)
Reinsurance cost synergies	$ 20,000
Reinsurance Multi-Year Limit	87,500
Quota Share Agreement, Percent Share | Rate						5.00%
Quota Share Agreement, Cession Rate | Rate			20.00%
Minimum
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
ISO Catastrophe Definition	$ 25,000
Unaffiliated Private Reinsurers [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Reinsurance Retention Policy, Number Of Private Reinsurers | reinsurer	43
Unaffiliated Private Reinsurers [Member] | Maximum [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Aggregate Coverage for Covered Losses Under Reinsurance Contract	$ 1,928,000
UPC, Subsidiary
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Quota Share Agreement, Percent Share | Rate						15.00%
FLORIDA | Florida Hurricane Catastrophe Fund [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Reinsurance Retention Policy, Participation Rate		45.00%
Aggregate Coverage for Covered Losses Under Reinsurance Contract		$ 789,000
First Catastrophic Event [Member] | Maximum [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Reinsurance Retention Policy, Amount Retained	55,000
Second And Subsequent Catastrophic Events [Member] | Maximum [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	2,747,500
Reinsurance Retention Policy, Amount Retained	30,000
Second And Subsequent Catastrophic Events [Member] | BlueLine [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Reinsurance Retention Policy, Amount Retained	$ 5,000