Relationship with Bayview Lending Group LLC and Bayview Financial Holdings, L.P. - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2020
Schedule of Equity Method Investments [Line Items]
Loan facility carrying amount	$ 4,382,193,000	$ 6,986,186,000
Bayview Lending Group [Member]
Schedule of Equity Method Investments [Line Items]
Minority interest in Bayview Lending Group LLC	20.00%
Carrying value of minority interest investment in Bayview Lending Group LLC	$ 0
Bayview Lending Group [Member] | Other Revenues From Operations [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	53,000,000	37,000,000	$ 24,000,000
Bayview Lending Group and Bayview Financial [Member]
Schedule of Equity Method Investments [Line Items]
Outstanding principal balances of mortgage servicing rights	1,900,000,000	2,200,000,000
Revenue from contract with customer	10,000,000	12,000,000	14,000,000
Bayview Financial [Member]
Schedule of Equity Method Investments [Line Items]
Outstanding principal balances of residential mortgage loans from Bayview Financial	68,100,000,000	62,800,000,000
Revenues from sub-servicing	129,000,000	125,000,000	$ 114,000,000
Investment securities in held-to-maturity portfolio securitized by Bayview Financial	77,000,000	$ 93,000,000
Secured loan facilities carrying amount				$ 100,000,000
Outstanding balance of secured debt				$ 60,000,000
Bayview Financial [Member] | FHA Guaranteed Mortgage Loans [Member]
Schedule of Equity Method Investments [Line Items]
FHA gauranteed mortgage loan	965,000,000
FHA gauranteed purchase amount of deliquent loan	1,000,000,000.0
Bayview Financial [Member] | Syndicated Loan Facility [Member]
Schedule of Equity Method Investments [Line Items]
Loan facility carrying amount	1,200,000,000
Bayview Financial [Member] | Syndicated Loan Facility [Member] | M&T Bank [Member]
Schedule of Equity Method Investments [Line Items]
Loan facility carrying amount	$ 210,000,000