Capitalized Servicing Assets - Changes in Capitalized Servicing Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Residential Mortgage Loans [Member]
Servicing Asset at Amortized Cost [Line Items]
Amortized value, Beginning balance	$ 244,411	$ 120,509	$ 114,978
Originations	45,101	26,067	28,985
Purchases		144,326	454
Amortization	(58,308)	(46,491)	(23,908)
Amortized value, ending balance	231,204	244,411	120,509
Valuation allowance	(30,000)	(7,000)
Ending balance, net	201,204	237,411	120,509
Commercial Mortgage Loans [Member]
Servicing Asset at Amortized Cost [Line Items]
Amortized value, Beginning balance	130,636	114,663	114,076
Originations	29,306	41,370	26,298
Amortization	(26,513)	(25,397)	(25,711)
Amortized value, ending balance	133,429	130,636	114,663
Ending balance, net	$ 133,429	$ 130,636	$ 114,663