Parent Company Only Financial Information - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Trading securities	$ 7,150	$ 4,858
Total Assets	2,768,773	2,573,821
Liabilities
Subordinated debentures	70,620	70,620
Total liabilities	2,524,863	2,362,037
Total shareholders' equity	243,910	211,784	$ 201,913	$ 188,548
Total Liabilities and Shareholders' Equity	2,768,773	2,573,821
Parent Company [Member]
ASSETS
Cash	1,556	1,436
Trading securities	580	529
Investment in bank subsidiary	290,156	257,476
Investment in non-bank subsidiaries	21,781	23,324
Deferreds and Current Receivable	2,055	1,066
Other assets	2,059	2,666
Total Assets	318,187	286,497
Liabilities
Borrowings from subsidiary	1,400	1,400
Subordinated debentures	70,620	70,620
Other liabilities	2,257	2,693
Total liabilities	74,277	74,713
Total shareholders' equity	243,910	201,913
Total Liabilities and Shareholders' Equity	$ 318,187	$ 286,497