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Investments - Unrealized Loss Position (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Investments    
Less Than 12 Months, Fair Value $ 9,994  
Less Than 12 Months, Unrealized Losses 2  
12 Months or Greater, Fair Value 5,667  
12 Months or Greater, Unrealized Losses 400  
Less Than 12 Months, Fair Value   $ 36,648
Less Than 12 Months, Unrealized Losses   2,050
12 Months or Greater, Fair Value   35,832
12 Months or Greater, Unrealized Losses   1,157
Unrestricted available-for-sale, Debt securities | U.S. Treasury securities    
Investments    
Less Than 12 Months, Fair Value 9,994  
Less Than 12 Months, Unrealized Losses 2  
Less Than 12 Months, Fair Value   9,870
Less Than 12 Months, Unrealized Losses   22
Unrestricted available-for-sale, Debt securities | Corporate debt securities    
Investments    
12 Months or Greater, Fair Value 2,405  
12 Months or Greater, Unrealized Losses 387  
Less Than 12 Months, Fair Value   15,515
Less Than 12 Months, Unrealized Losses   691
12 Months or Greater, Fair Value   29,595
12 Months or Greater, Unrealized Losses   1,126
Equity securities | Preferred stock    
Investments    
Less Than 12 Months, Fair Value   11,263
Less Than 12 Months, Unrealized Losses   1,337
12 Months or Greater, Fair Value   1,986
12 Months or Greater, Unrealized Losses   18
Restricted | Short-term bond    
Investments    
12 Months or Greater, Fair Value 3,262  
12 Months or Greater, Unrealized Losses $ 13  
12 Months or Greater, Fair Value   4,251
12 Months or Greater, Unrealized Losses   $ 13