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Financial Instruments Financial Instruments narrative (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Document Period End Date Mar. 31, 2020  
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value $ 989,934 $ 1,040,678
Debt securities gross unrealized gains 2,400  
Debt instrument fair value senior notes 2023 751,600  
Debt instrument fair value senior notes 2025 511,600  
Debt instrument fair value senior notes 2027 $ 575,000