Basis of Presentation and Significant Accounting Policies Measurement of Credit Losses on Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Basis of Presentation and Significant Accounting Policies [Abstract]
Available-for-sale securities, amortized cost	$ 17,030,405			$ 30,711,157
Available-for-sale securities, allowance for credit losses	$ 42,583	$ 41,390	$ 244,876	$ 0