Financing Agreements (Borrowings Under Repurchase Agreement) (Details) - Repurchase Agreement Borrowings - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Other financing agreements	$ 3,213,056	$ 9,140,944
Weighted Average Interest Rate	3.33%	2.99%
Less debt issuance costs	$ 0	$ 1,123
Total financing agreements less debt issuance costs	3,213,056	9,139,821
Within 30 days
Debt Securities, Available-for-sale [Line Items]
Other financing agreements	$ 2,932,213	$ 4,472,120
Weighted Average Interest Rate	3.36%	2.55%
Over 30 days to 3 months
Debt Securities, Available-for-sale [Line Items]
Other financing agreements	$ 0	$ 2,746,384
Weighted Average Interest Rate	0.00%	3.43%
Over 3 months to 12 months
Debt Securities, Available-for-sale [Line Items]
Other financing agreements	$ 280,843	$ 1,014,441
Weighted Average Interest Rate	3.02%	3.36%
Over 12 months
Debt Securities, Available-for-sale [Line Items]
Other financing agreements	$ 0	$ 907,999
Weighted Average Interest Rate	0.00%	3.44%