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Transfers of Residential Loans - Additional Information (Detail) - USD ($)
$ in Billions
1 Months Ended 11 Months Ended 12 Months Ended
Feb. 09, 2018
Dec. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Mortgage loans sold and serviced, percent past due   3.00% 3.00% 2.90%
Customer Concentration Risk [Member] | Mortgage Loans Sold [Member] | Ginnie Mae [Member]        
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Concentration percentage 54.00% 57.00%   41.00%
Customer Concentration Risk [Member] | Mortgage Loans Sold [Member] | Fannie Mae [Member]        
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Concentration percentage 39.00% 28.00%   50.00%
Customer Concentration Risk [Member] | Mortgage Loans Sold [Member] | Freddie Mac [Member]        
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Concentration percentage 7.00% 15.00%   9.00%
Residential Loan Transfers Accounted for as Sales [Member]        
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Mortgage loans sold and serviced, number of days past due threshold     60 days or more  
Reverse Loans And Real Estate Owned [Member]        
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Unpaid principal balance of assets pledged as collateral to securitization pools   $ 6.8 $ 6.8  
Carrying value of assets pledged as collateral to securitzation pools   $ 7.1 $ 7.1