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Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
fund
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities $ 748,523,000 $ 745,961,000  
Equity securities 11,853,000    
Total available-for-sale securities   758,713,000  
Total 763,582,000 763,529,000  
Other limited partnership investments:      
Contributions made to the funds 243,000 410,000 $ 578,000
Distributions received from the funds $ 3,387,000 10,768,000 $ 16,113,000
Number of real estate funds reported under the fair value option | fund 1    
U.S. Treasury      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities $ 208,412,000 11,734,000  
States and political subdivisions      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 159,023,000 259,264,000  
Foreign government securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities   503,000  
Corporate debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 249,947,000 346,523,000  
Residential mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 4,609,000 25,571,000  
Commercial mortgage-backed securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 46,515,000 32,804,000  
Collateralized debt obligations      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 64,239,000 58,034,000  
Other debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 15,778,000 11,528,000  
Nonredeemable preferred stock | Financial Services Sector      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale equity securities   11,659,000  
Nonredeemable preferred stock | Utilities Sector      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale equity securities   1,093,000  
Quoted prices in active markets for identical assets Level 1      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 0 0  
Equity securities 1,809,000    
Total available-for-sale securities   2,015,000  
Total 1,809,000 2,015,000  
Observable inputs Level 2      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 735,946,000 735,882,000  
Equity securities 10,044,000    
Total available-for-sale securities   746,619,000  
Total 745,990,000 746,619,000  
Unobservable inputs Level 3      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 12,577,000 10,079,000  
Equity securities 0    
Total available-for-sale securities   10,079,000  
Total 12,577,000 10,079,000  
Fair Value, Measurements, Recurring Basis | U.S. Treasury      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 208,412,000 11,734,000  
Fair Value, Measurements, Recurring Basis | States and political subdivisions      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 159,023,000 259,264,000  
Fair Value, Measurements, Recurring Basis | Foreign government securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities   503,000  
Fair Value, Measurements, Recurring Basis | Corporate debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 249,947,000 346,523,000  
Fair Value, Measurements, 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 4,609,000 25,571,000  
Fair Value, Measurements, 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 46,515,000 32,804,000  
Fair Value, Measurements, Recurring Basis | Collateralized debt obligations      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 64,239,000 58,034,000  
Fair Value, Measurements, Recurring Basis | Other debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 15,778,000 11,528,000  
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Financial Services Sector      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 11,853,000    
Available-for-sale equity securities   11,659,000  
Fair Value, Measurements, 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   1,093,000  
Fair Value, Measurements, Recurring Basis | Real estate      
Other limited partnership investments:      
Contributions made to the funds 0 0  
Distributions received from the funds 1,200,000 500,000  
Unfunded commitments $ 0 0  
Number of real estate funds reported under the fair value option | fund 1    
Fair Value, Measurements, Recurring Basis | Quoted prices in active markets for identical assets 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, Recurring Basis | Quoted prices in active markets for identical assets Level 1 | States and 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, Recurring Basis | Quoted prices in active markets for identical assets 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  
Fair Value, Measurements, Recurring Basis | Quoted prices in active markets for identical assets 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, Recurring Basis | Quoted prices in active markets for identical assets 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, Recurring Basis | Quoted prices in active markets for identical assets 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, Recurring Basis | Quoted prices in active markets for identical assets 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, Recurring Basis | Quoted prices in active markets for identical assets 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, Recurring Basis | Quoted prices in active markets for identical assets Level 1 | Nonredeemable preferred stock | Financial Services Sector      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 1,809,000    
Available-for-sale equity securities   2,015,000  
Fair Value, Measurements, Recurring Basis | Quoted prices in active markets for identical assets 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, Recurring Basis | Observable inputs Level 2 | U.S. Treasury      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 208,412,000 11,734,000  
Fair Value, Measurements, Recurring Basis | Observable inputs Level 2 | States and political subdivisions      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 159,023,000 259,264,000  
Fair Value, Measurements, Recurring Basis | Observable inputs Level 2 | Foreign government securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities   503,000  
Fair Value, Measurements, Recurring Basis | Observable inputs Level 2 | Corporate debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 237,370,000 338,644,000  
Fair Value, Measurements, Recurring Basis | Observable inputs 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 4,609,000 25,571,000  
Fair Value, Measurements, Recurring Basis | Observable inputs 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 46,515,000 32,804,000  
Fair Value, Measurements, Recurring Basis | Observable inputs Level 2 | Collateralized debt obligations      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 64,239,000 55,834,000  
Fair Value, Measurements, Recurring Basis | Observable inputs Level 2 | Other debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 15,778,000 11,528,000  
Fair Value, Measurements, Recurring Basis | Observable inputs Level 2 | Nonredeemable preferred stock | Financial Services Sector      
Fair value measurements on a recurring basis by asset class and level of input      
Equity securities 10,044,000    
Available-for-sale equity securities   9,644,000  
Fair Value, Measurements, Recurring Basis | Observable inputs 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   1,093,000  
Fair Value, Measurements, Recurring Basis | Unobservable inputs 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, Recurring Basis | Unobservable inputs Level 3 | States and 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, Recurring Basis | Unobservable inputs 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  
Fair Value, Measurements, Recurring Basis | Unobservable inputs Level 3 | Corporate debt securities      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale debt securities 12,577,000 7,879,000  
Fair Value, Measurements, Recurring Basis | Unobservable inputs 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, Recurring Basis | Unobservable inputs 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, Recurring Basis | Unobservable inputs 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 2,200,000  
Fair Value, Measurements, Recurring Basis | Unobservable inputs 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, Recurring Basis | Unobservable inputs 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, Recurring Basis | Unobservable inputs 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  
Fair Value, Measurements, Recurring Basis | Fair value option using net asset value (NAV) | Real estate      
Fair value measurements on a recurring basis by asset class and level of input      
Other limited partnership investments $ 3,206,000 $ 4,816,000