Noninterest Income and Expense - Components of Other Noninterest Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Other Noninterest Income And Expense [Line Items]
Total service charges and fees									$ 38,460	$ 37,423	$ 33,226
Commissions on sale of non-deposit investment products									3,151	2,729	2,329
Increase in cash value of life insurance									2,718	2,685	2,717
Gain on sale of loans									2,371	3,109	4,037
Lease brokerage income									678	782	711
Sale of customer checks									449	372	408
Gain on sale of foreclosed assets									408	711	262
Gain on sale of investment securities									207	961
Loss on disposal of fixed assets									(185)	(142)	(147)
Loss on marketable equity securities									(64)
Other									1,091	1,391	1,020
Total other noninterest income									10,824	12,598	11,337
Total noninterest income	$ 12,634	$ 12,186	$ 12,174	$ 12,290	$ 12,478	$ 12,930	$ 12,910	$ 11,703	49,284	50,021	44,563
ATM and Interchange Fees [Member]
Schedule Of Other Noninterest Income And Expense [Line Items]
Total service charges and fees									18,249	16,727	15,859
Service Charges on Deposit Accounts [Member]
Schedule Of Other Noninterest Income And Expense [Line Items]
Total service charges and fees									15,467	16,056	14,365
Other Service Fees [Member]
Schedule Of Other Noninterest Income And Expense [Line Items]
Total service charges and fees									2,852	3,282	3,121
Mortgage Banking Service Fees [Member]
Schedule Of Other Noninterest Income And Expense [Line Items]
Total service charges and fees									2,038	2,076	2,065
Change in Value of Mortgage Servicing Rights [Member]
Schedule Of Other Noninterest Income And Expense [Line Items]
Total service charges and fees									$ (146)	$ (718)	$ (2,184)