Repurchase Agreements Remaining Term to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 2,437,163	$ 4,752,348
Assets Sold under Agreements to Repurchase, Interest Rate	0.34%	2.01%
WAVG Original Term to Maturity	44	45
Less than 30 days
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 1,416,608	$ 2,078,185
Assets Sold under Agreements to Repurchase, Interest Rate	0.37%	2.12%
WAVG Original Term to Maturity	53	34
30 to 90 days
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 845,394	$ 2,674,163
Assets Sold under Agreements to Repurchase, Interest Rate	0.31%	1.93%
WAVG Original Term to Maturity	35	52
Maturity Greater than 90 Days
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 175,161
Assets Sold under Agreements to Repurchase, Interest Rate	0.22%
WAVG Original Term to Maturity	13