Loans and Other Real Estate - Company's Commercial Loan Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4,883,412	$ 4,419,045
Commercial, secured by real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,436,550	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	68,756	69,995
Commercial, secured by real estate | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	933,390	933,577
Commercial, secured by real estate | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,247,650	1,910,423
Commercial, secured by real estate | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	89,295	61,626
Commercial, secured by real estate | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	46,466	38,844
Commercial, secured by real estate | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	50,993	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	389,230	336,735
Commercial, industrial and other | 1
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,450	1,119
Commercial, industrial and other | 2
Financing Receivable, Recorded Investment [Line Items]
Total loans	18,444	18,462
Commercial, industrial and other | 3
Financing Receivable, Recorded Investment [Line Items]
Total loans	36,739	36,367
Commercial, industrial and other | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	88,551	91,145
Commercial, industrial and other | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	205,999	168,474
Commercial, industrial and other | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	19,381	7,798
Commercial, industrial and other | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,988	2,033
Commercial, industrial and other | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	13,678	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	332,995	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	18,204	17,375
Real estate - construction | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	302,190	297,625
Real estate - construction | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,873	3,493
Real estate - construction | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,267	0
Real estate - construction | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,461	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