Mortgage Banking Activities - Principal Balances of Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 1,500,000	$ 1,400,000
FHLMC [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	1,002,212	1,016,097
FNMA [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	451,762	347,294
Private Investor [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 10,640	$ 15,049