Residential Mortgage Securities and MSR-Related Assets (Credit Losses Rollforward) (Details) - Mortgage-backed Securities, Issued by US Government Sponsored Enterprises - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Allowance for credit losses at beginning of period	$ 0	$ 0
Securities with no prior loss allowance	332,756,000	0
Securities with a prior loss allowance	0	0
Write-offs, including allowance related to securities we intend to sell	(332,756,000)	0
Allowance for credit losses at end of period	$ 0	$ 0