Financing Agreements (Borrowings Under Repurchase Agreement) (Details) - Repurchase Agreement Borrowings - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 1,964,377	$ 2,494,976
Weighted Average Interest Rate	2.55%	3.16%
Carrying Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 1,313,747
Weighted Average Interest Rate	2.00%
Within 30 days | Fair Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 1,964,377	$ 2,494,976
Weighted Average Interest Rate	2.55%	3.16%
Within 30 days | Carrying Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 1,259,588
Weighted Average Interest Rate	1.90%
Over 30 days to 3 months | Fair Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 0	$ 0
Weighted Average Interest Rate	0.00%	0.00%
Over 30 days to 3 months | Carrying Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 36,141
Weighted Average Interest Rate	4.30%
Over 3 months to 12 months | Fair Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 0	$ 0
Weighted Average Interest Rate	0.00%	0.00%
Over 3 months to 12 months | Carrying Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 18,018
Weighted Average Interest Rate	4.50%
Over 12 months | Fair Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 0	$ 0
Weighted Average Interest Rate	0.00%	0.00%
Over 12 months | Carrying Value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 0
Weighted Average Interest Rate	0.00%