Repurchase Agreements Remaining Term to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ 2,594,683	$ 4,752,348
Assets Sold under Agreements to Repurchase, Interest Rate	0.36%	2.01%
WAVG Original Term to Maturity	71	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	$ 2,048,541	$ 2,078,185
Assets Sold under Agreements to Repurchase, Interest Rate	0.37%	2.12%
WAVG Original Term to Maturity	64	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	$ 496,755	$ 2,674,163
Assets Sold under Agreements to Repurchase, Interest Rate	0.27%	1.93%
WAVG Original Term to Maturity	95	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	$ 49,387
Assets Sold under Agreements to Repurchase, Interest Rate	0.93%
WAVG Original Term to Maturity	121