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Reinsurance - Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Jul. 01, 2015
Mar. 31, 2018
Sep. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Effects of Reinsurance [Line Items]              
Ceded premiums written, net of profit commission       $ 125,726,000 $ 133,885,000 $ 55,391,000  
Ceded to Other Companies       125,735,000 133,981,000 102,848,000  
Reinsurance recoverable on loss reserves       $ 48,474,000 $ 50,493,000 44,487,000 $ 57,841,000
Period of existing captive reinsurance agreement       10 years 10 years    
Fair value of trust fund assets under our captive agreements       $ 80,000,000 $ 91,000,000    
Quota Share Reinsurance Agreement, 2017              
Effects of Reinsurance [Line Items]              
Contingent termination fee       $ 0      
Threshold for private mortgage insurer eligibility requirements for termination election       90.00%      
Quota share, percentage       30.00%      
Ceding commission, percentage       20.00%      
Loss ratio       60.00%      
Quota Share Reinsurance Agreement, 2015              
Effects of Reinsurance [Line Items]              
Contingent termination fee       $ 0      
Threshold for private mortgage insurer eligibility requirements for termination election       90.00%      
Quota share, percentage       30.00%      
Ceding commission, percentage       20.00%      
Loss ratio       60.00%      
Quota Share Reinsurance Agreement, 2013              
Effects of Reinsurance [Line Items]              
Ceded premiums written, net of profit commission     $ 69,400,000        
Ceded to Other Companies     11,600,000        
Ceding commissions     $ 11,600,000        
Profit commission $ 142,500,000            
Quota Share Reinsurance Agreement, 2018 | Forecast              
Effects of Reinsurance [Line Items]              
Contingent termination fee   $ 0          
Threshold for private mortgage insurer eligibility requirements for termination election   90.00%          
Quota share, percentage   30.00%          
Ceding commission, percentage   20.00%          
Loss ratio   54.00%          
Quota Share Reinsurance Agreements, Excluding Captives              
Effects of Reinsurance [Line Items]              
Ceded premiums written, net of profit commission       $ 120,974,000 125,460,000 41,233,000  
Ceded to Other Companies       120,974,000 125,460,000 88,587,000  
Ceding commissions       49,321,000 47,629,000 30,816,000  
Profit commission       125,629,000 112,685,000 $ 112,847,000  
Reinsurance recoverable on loss reserves       39,300,000 31,800,000    
Reinsurance Agreements, Captive Agreements              
Effects of Reinsurance [Line Items]              
Reinsurance recoverable on loss reserves       $ 9,000,000 $ 19,000,000