Loans and Other Real Estate - Company's Commercial Loan Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4,884,759	$ 4,419,045
Commercial, secured by real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,442,973	3,057,779
Commercial, secured by real estate | 1
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial, secured by real estate | 2
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial, secured by real estate | 3
Financing Receivable, Recorded Investment [Line Items]
Total loans	71,581	69,995
Commercial, secured by real estate | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	910,695	933,577
Commercial, secured by real estate | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,245,570	1,910,423
Commercial, secured by real estate | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	103,619	61,626
Commercial, secured by real estate | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	50,155	38,844
Commercial, secured by real estate | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	61,353	43,314
Commercial, secured by real estate | 8 - Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial, secured by real estate | 9 - Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial, industrial and other
Financing Receivable, Recorded Investment [Line Items]
Total loans	391,486	336,735
Commercial, industrial and other | 1
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,655	1,119
Commercial, industrial and other | 2
Financing Receivable, Recorded Investment [Line Items]
Total loans	17,865	18,462
Commercial, industrial and other | 3
Financing Receivable, Recorded Investment [Line Items]
Total loans	34,392	36,367
Commercial, industrial and other | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	91,329	91,145
Commercial, industrial and other | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	208,024	168,474
Commercial, industrial and other | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	24,302	7,798
Commercial, industrial and other | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,805	2,033
Commercial, industrial and other | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	9,114	11,337
Commercial, industrial and other | 8 - Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial, industrial and other | 9 - Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate - construction
Financing Receivable, Recorded Investment [Line Items]
Total loans	306,440	319,545
Real estate - construction | 1
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate - construction | 2
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate - construction | 3
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate - construction | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	18,871	17,375
Real estate - construction | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	273,997	297,625
Real estate - construction | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,487	3,493
Real estate - construction | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	967	0
Real estate - construction | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,118	1,052
Real estate - construction | 8 - Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate - construction | 9 - Loss
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 0