Residential Whole Loans - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) loan	Mar. 31, 2021 loan	Dec. 31, 2021 USD ($)
Receivables [Abstract]
Total residential whole loans	$ 8,300.0		$ 7,900.0
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of whole loans sold | loan	0	0
Nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	$ 550.8		588.1
Purchased Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	221.1		240.2
Purchased Credit Deteriorated Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	101.2		108.9
Interest Income On Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	4.3		3.0
Without Associated Credit Losses
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	$ 107.2		$ 107.4