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Fair Value (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities $ 729,804    
Equity securities 12,583    
Total available-for-sale securities 729,804   $ 758,713
Other investments:      
Contributions made to the funds 31 $ 111  
Distributions received from the funds 910 $ 3,396  
Fair value measurements on a recurring basis      
Fair value measurements on a recurring basis by asset class and level of input      
Total available-for-sale securities     758,713
Total 746,816   763,529
Fair value measurements on a recurring basis | Fixed maturities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 729,804   745,961
Fair value measurements on a recurring basis | U.S. treasury      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 36,464   11,734
Fair value measurements on a recurring basis | States & political subdivisions      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 261,152   259,264
Fair value measurements on a recurring basis | Foreign government securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 501   503
Fair value measurements on a recurring basis | Corporate debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 299,599   346,523
Fair value measurements on a recurring basis | Residential mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 24,110   25,571
Fair value measurements on a recurring basis | Commercial mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 33,675   32,804
Fair value measurements on a recurring basis | Collateralized debt obligations      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 71,305   58,034
Fair value measurements on a recurring basis | Other debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 2,998   11,528
Fair value measurements on a recurring basis | Equity securities      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 12,583    
Fair value measurements on a recurring basis | Nonredeemable preferred stock | Financial services sector      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 12,583    
Available-for-sale equity securities     11,659
Fair value measurements on a recurring basis | Nonredeemable preferred stock | Utilities sector      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale equity securities     1,093
Fair value measurements on a recurring basis | Real estate      
Fair value measurements on a recurring basis by asset class and level of input      
Other investments 4,429   4,816
Other investments:      
Contributions made to the funds 0   0
Distributions received from the funds 0   500
Unfunded commitments $ 0   $ 0
Fair value measurements on a recurring basis | Real estate | Minimum      
Other investments:      
Underlying assets, liquidation period from inception of fund (in years) 5 years   5 years
Fair value measurements on a recurring basis | Real estate | Maximum      
Other investments:      
Underlying assets, liquidation period from inception of fund (in years) 10 years   10 years
Fair value measurements on a recurring basis | Level 1      
Fair value measurements on a recurring basis by asset class and level of input      
Total available-for-sale securities     $ 2,015
Total $ 1,993   2,015
Fair value measurements on a recurring basis | Level 1 | Fixed maturities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | U.S. treasury      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | States & political subdivisions      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | Foreign government securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | Corporate debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | Residential mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | Commercial mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | Collateralized debt obligations      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | Other debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 1 | Equity securities      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 1,993    
Fair value measurements on a recurring basis | Level 1 | Nonredeemable preferred stock | Financial services sector      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 1,993    
Available-for-sale equity securities     2,015
Fair value measurements on a recurring basis | Level 1 | Nonredeemable preferred stock | Utilities sector      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale equity securities     0
Fair value measurements on a recurring basis | Level 2      
Fair value measurements on a recurring basis by asset class and level of input      
Total available-for-sale securities     746,619
Total 734,085   746,619
Fair value measurements on a recurring basis | Level 2 | Fixed maturities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 723,495   735,882
Fair value measurements on a recurring basis | Level 2 | U.S. treasury      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 36,464   11,734
Fair value measurements on a recurring basis | Level 2 | States & political subdivisions      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 261,152   259,264
Fair value measurements on a recurring basis | Level 2 | Foreign government securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 501   503
Fair value measurements on a recurring basis | Level 2 | Corporate debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 293,290   338,644
Fair value measurements on a recurring basis | Level 2 | Residential mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 24,110   25,571
Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 33,675   32,804
Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 71,305   55,834
Fair value measurements on a recurring basis | Level 2 | Other debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 2,998   11,528
Fair value measurements on a recurring basis | Level 2 | Equity securities      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 10,590    
Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock | Financial services sector      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 10,590    
Available-for-sale equity securities     9,644
Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock | Utilities sector      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale equity securities     1,093
Fair value measurements on a recurring basis | Level 3      
Fair value measurements on a recurring basis by asset class and level of input      
Total available-for-sale securities     10,079
Total 6,309   10,079
Fair value measurements on a recurring basis | Level 3 | Fixed maturities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 6,309   10,079
Fair value measurements on a recurring basis | Level 3 | U.S. treasury      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 3 | States & political subdivisions      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 3 | Foreign government securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 3 | Corporate debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 6,309   7,879
Fair value measurements on a recurring basis | Level 3 | Residential mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 3 | Commercial mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 3 | Collateralized debt obligations      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   2,200
Fair value measurements on a recurring basis | Level 3 | Other debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0   0
Fair value measurements on a recurring basis | Level 3 | Equity securities      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 0    
Fair value measurements on a recurring basis | Level 3 | Nonredeemable preferred stock | Financial services sector      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities $ 0    
Available-for-sale equity securities     0
Fair value measurements on a recurring basis | Level 3 | Nonredeemable preferred stock | Utilities sector      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale equity securities     $ 0