Noninterest Income and Expense - Components of Other Noninterest Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Service charges on deposit accounts									$ 16,056	$ 14,365	$ 14,276
ATM and interchange fees									16,727	15,859	13,364
Other service fees									3,282	3,121	2,977
Mortgage banking service fees									2,076	2,065	2,164
Change in value of mortgage servicing rights									(718)	(2,184)	(701)
Total service charges and fees									37,423	33,226	32,080
Gain on sale of loans									3,109	4,037	3,064
Commissions on sale of non-deposit investment products									2,729	2,329	3,349
Increase in cash value of life insurance									2,685	2,717	2,786
Gain on sale of investments									961		0
Lease brokerage income									782	711	712
Gain on sale of foreclosed assets									711	262	991
Change in indemnification asset									490	(493)	(207)
Sale of customer checks									372	408	492
Life insurance benefit in excess of cash value									108	238	155
Loss on disposal of fixed assets									(142)	(147)	(129)
Other									793	1,275	2,054
Total other noninterest income									12,598	11,337	13,267
Total noninterest income	$ 12,478	$ 12,930	$ 12,910	$ 11,703	$ 12,462	$ 11,066	$ 11,245	$ 9,790	$ 50,021	$ 44,563	$ 45,347