Derivative Financial Instruments - Offsetting of Derivative Assets - Offsetting of Derivative and Financial Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	$ 277,337	$ 252,425
Netting	(233,763)	(212,722)
Net amounts of liabilities presented in the balance sheet	43,574	39,703
Total
Gross amounts of recognized liabilities	302,320	255,360
Net amounts of liabilities presented in the consolidated balance sheet	68,557	42,638
Mortgage loans sold under agreements to repurchase
Net amounts of liabilities presented in the consolidated balance sheet	10,856,677
Debt Issuance Costs
Debt issuance costs, gross	(8,200)	(9,198)
Debt issuance costs	(8,200)	(9,198)
Gross amounts of recognized liabilities	10,848,477	9,654,797
Net amount of liabilities in the consolidated balance sheet	10,848,477	9,654,797
Total
Gross amounts of recognized liabilities	11,150,797	9,910,157
Gross amounts offset in the consolidated balance sheet	(233,763)	(212,722)
Net amounts of liabilities presented in the consolidated balance sheet	10,925,234	9,706,633
Net amount of liabilities in the consolidated balance sheet	68,557	42,638
Receivable from Counterparties
Total
Net amounts of liabilities presented in the consolidated balance sheet	10,917,034	9,697,435
Assets sold under agreements to repurchase
Mortgage loans sold under agreements to repurchase
Gross amounts of recognized liabilities	10,856,677	9,663,995
Net amounts of liabilities presented in the consolidated balance sheet	10,856,677	9,663,995
Net amounts of liabilities presented in the consolidated balance sheet	10,856,677	9,663,995
Debt Issuance Costs
Debt issuance costs	(8,200)	(9,198)
Net amount of liabilities in the consolidated balance sheet	10,848,477	9,654,797
Forward contracts | Purchases
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	192,793	1,276
Net amounts of liabilities presented in the balance sheet	192,793	1,276
Total
Gross amounts of recognized liabilities	192,793	1,276
Forward contracts | Sales
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	32,729	251,149
Net amounts of liabilities presented in the balance sheet	32,729	251,149
Total
Gross amounts of recognized liabilities	32,729	251,149
Interest rate lock commitments
Derivatives not subject to master netting arrangements
Gross amounts of recognized liabilities	24,983	2,935
Total
Net amounts of liabilities presented in the consolidated balance sheet	24,983	2,935
Net amount of liabilities in the consolidated balance sheet	24,983	$ 2,935
MBS put options
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	34,873
Net amounts of liabilities presented in the balance sheet	34,873
Total
Gross amounts of recognized liabilities	34,873
Put options on interest rate futures | Sales
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	15,881
Net amounts of liabilities presented in the balance sheet	15,881
Total
Gross amounts of recognized liabilities	15,881
Call options on interest rate futures | Sales
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	1,061
Net amounts of liabilities presented in the balance sheet	1,061
Total
Gross amounts of recognized liabilities	$ 1,061