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Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 2,168,885  
Estimated Fair Value 2,159,806 $ 2,080,294
Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 122,006  
Estimated Fair Value $ 121,288