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Investments - Held-to-maturity Security - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Schedule Of Held To Maturity Securities [Line Items]    
Debt instrument, maturity date   Jul. 15, 2022
Debt instrument, interest rate, stated percentage   4.75%
Held-to-maturity debt security estimated unrealized holding loss $ (25.1)  
LLC Note [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Debt instrument, maturity date Dec. 31, 2030  
Debt instrument, interest rate, stated percentage 4.50%  
Surplus Note [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Debt instrument, maturity date Dec. 31, 2030  
Debt instrument, interest rate, stated percentage 4.50%