Loan Servicing - Mortgage Servicing Rights Activity (Detail) (Details) - Mortgage Loan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Servicing Assets at Fair Value [Line Items]
SBA loans sold to investors with servicing retained	$ 3,256,548	$ 1,800,002
Mortgage loans sold with servicing retained	1,769,129	672,536	$ 431,617
Pretax gains resulting from above loan sales	21,683	12,519	10,681
Mortgage servicing fees	7,509	3,803	3,056
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	13,521	8,921	6,859
Fair value of MSRs acquired	8,553	0	0
Additions	10,830	7,148	3,370
Payoffs and curtailments	(1,491)	(780)	(694)
Amortization	(2,360)	(1,768)	(614)
Balance at end of period	29,053	13,521	8,921
Fair value, beginning of period	17,546	11,503	8,684
Fair value, end of period	$ 32,419	$ 17,546	$ 11,503