Mortgage Loans Held for Sale and Investment - Mortgage Loans Held For Investment - UPB, Fair Value (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Mortgage Loans Held For Investment [Roll Forward]
Balance - beginning of period	$ 119
Changes in fair value	3	$ 0
Balance - end of period	114
Mortgage loans held for investment, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
UPB	21		$ 27
Fair Value	10		$ 13
Mortgage Loans Held For Investment [Roll Forward]
Balance - beginning of period	119
Payments received from borrowers	(8)
Changes in fair value	3
Balance - end of period	$ 114