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Reinsurance - Narrative (Details) - USD ($)
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Effects of Reinsurance [Line Items]        
Reinsurance recoverable on loss reserves $ 45,878,000 $ 50,493,000 $ 46,863,000 $ 44,487,000
Period of existing captive reinsurance agreement 10 years      
Fair value of trust fund assets under 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 (less than) (as a percent) 90.00%      
Quota share for all policies covered (as a percent) 30.00%      
Ceding commission, percentage (as a percent) 20.00%      
Loss ratio threshold for profit commissions (as a percent) 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 (less than) (as a percent) 90.00%      
Quota share for all policies covered (as a percent) 30.00%      
Ceding commission, percentage (as a percent) 20.00%      
Loss ratio threshold for profit commissions (as a percent) 60.00%      
Quota Share Reinsurance Agreements, Excluding Captive Agreements        
Effects of Reinsurance [Line Items]        
Reinsurance recoverable on loss reserves $ 35,300,000 31,800,000    
Reinsurance Agreements, Captive Agreements        
Effects of Reinsurance [Line Items]        
Reinsurance recoverable on loss reserves related to captive agreements $ 11,000,000 $ 19,000,000