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Investments - Held-to-maturity Security - Narrative (Details)
$ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
Schedule Of Held To Maturity Securities [Line Items]  
Held-to-maturity debt security estimated unrealized holding loss $ 5.2
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%