Residential Mortgage Securities and MSR-Related Assets (Credit Losses Rollforward) (Details) - Mortgage-backed Securities, Issued by US Government Sponsored Enterprises - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Allowance for credit losses at beginning of period	$ 0	$ 0	$ 0	$ 0
Securities with no prior loss allowance	0	0	344,269,000	0
Securities with a prior loss allowance	0	0	0	0
Write-offs, including allowance related to securities the Company intends to sell	0	0	(344,269,000)	0
Allowance for credit losses at end of period	$ 0	$ 0	$ 0	$ 0