Financial Instruments Financial Instruments narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	$ 906,492	$ 1,013,679
Debt instrument fair value senior notes 2025	568,200
Debt instrument fair value senior notes 2027	586,500
Debt instrument fair value senior notes 2031	761,600
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	906,492	$ 1,013,679
Debt securities gross unrealized losses	$ 100