Mortgage Banking Activities - Principal Balances of Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 1,400,000	$ 1,300,000
FHLMC [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	1,007,223	1,003,441
FNMA [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	326,200	242,444
Private Investor [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 16,404	$ 23,404