Certain Transfers of Financial Assets - Asset Quality of Residential Mortgage Loans (Details) - Residential mortgage	12 Months Ended
	Dec. 31, 2017 USD ($)
Asset quality of mortgage loans
Unpaid Principal Balance	$ 9,674,928,000
Delinquent 30 to 89 days	156,464,000
Delinquent 90 plus Days	34,930,000
Net Charge-offs for the Year Ended December 31, 2017	(21,000)
Serviced for others
Asset quality of mortgage loans
Unpaid Principal Balance	8,924,808,000
Delinquent 30 to 89 days	145,059,000
Delinquent 90 plus Days	17,773,000
Net Charge-offs for the Year Ended December 31, 2017	0
Held-for-sale(1)
Asset quality of mortgage loans
Unpaid Principal Balance	262,315,000
Delinquent 30 to 89 days	0
Delinquent 90 plus Days	121,000
Net Charge-offs for the Year Ended December 31, 2017	0
Mortgage loans delinquent 90 Plus Days, at fair value	109,000
Held-for-investment
Asset quality of mortgage loans
Unpaid Principal Balance	487,805,000
Delinquent 30 to 89 days	11,405,000
Delinquent 90 plus Days	17,036,000
Net Charge-offs for the Year Ended December 31, 2017	(21,000)
Loans Insured or Guaranteed by US Government Authorities | Held-for-investment
Asset quality of mortgage loans
Delinquent 30 to 89 days	3,389,000
Delinquent 90 plus Days	$ 13,713