Loans, Leases and Other Real Estate - Company's Commercial Loan Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4,290,825	$ 4,117,265
Commercial, secured by real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,984,430	2,831,184
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	70,548	76,824
Commercial, secured by real estate | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	903,866	862,537
Commercial, secured by real estate | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,886,023	1,779,908
Commercial, secured by real estate | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	46,537	47,178
Commercial, secured by real estate | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	41,237	40,245
Commercial, secured by real estate | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	36,219	24,492
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	334,241	340,400
Commercial, industrial and other | 1
Financing Receivable, Recorded Investment [Line Items]
Total loans	279	392
Commercial, industrial and other | 2
Financing Receivable, Recorded Investment [Line Items]
Total loans	17,907	26,968
Commercial, industrial and other | 3
Financing Receivable, Recorded Investment [Line Items]
Total loans	42,177	35,950
Commercial, industrial and other | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	84,973	96,426
Commercial, industrial and other | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	163,114	150,928
Commercial, industrial and other | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,428	8,779
Commercial, industrial and other | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,444	8,670
Commercial, industrial and other | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,919	12,287
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	297,516	264,908
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	28,780	15,502
Real estate - construction | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	266,054	246,806
Real estate - construction | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,611	0
Real estate - construction | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate - construction | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,071	2,600
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