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Concentration of Risks and Uncertainties (Details)
$ in Thousands
Jun. 30, 2016
USD ($)
issuer
Concentration Risk [Line Items]  
Corporate debt securities $ 193,576
U.S. Treasury Securities  
Concentration Risk [Line Items]  
U.S. Treasury securities 174,600
Corporate debt securities  
Concentration Risk [Line Items]  
Corporate debt securities 19,000
Preferred stock  
Concentration Risk [Line Items]  
Preferred stock investments $ 22,700
Commercial paper  
Concentration Risk [Line Items]  
Number of issuers | issuer 6
Commercial paper $ 139,100
Non-investment grade | Corporate debt securities  
Concentration Risk [Line Items]  
Corporate debt securities $ 19,000
Number of issuers | issuer 5
Non-investment grade | Preferred stock  
Concentration Risk [Line Items]  
Number of issuers | issuer 2
Preferred stock investments $ 18,900
Investment grade | Preferred stock  
Concentration Risk [Line Items]  
Number of issuers | issuer 1
Preferred stock investments $ 3,800