Loans, Leases and Other Real Estate - Company's Commercial Loan Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4,188,325	$ 4,117,265
Commercial, secured by real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,885,997	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	75,608	76,824
Commercial, secured by real estate | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	882,811	862,537
Commercial, secured by real estate | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,822,829	1,779,908
Commercial, secured by real estate | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	40,360	47,178
Commercial, secured by real estate | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	41,753	40,245
Commercial, secured by real estate | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	22,636	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	339,665	340,400
Commercial, industrial and other | 1
Financing Receivable, Recorded Investment [Line Items]
Total loans	362	392
Commercial, industrial and other | 2
Financing Receivable, Recorded Investment [Line Items]
Total loans	26,755	26,968
Commercial, industrial and other | 3
Financing Receivable, Recorded Investment [Line Items]
Total loans	44,973	35,950
Commercial, industrial and other | 4
Financing Receivable, Recorded Investment [Line Items]
Total loans	96,031	96,426
Commercial, industrial and other | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	145,396	150,928
Commercial, industrial and other | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	9,022	8,779
Commercial, industrial and other | 6 - Other assets especially mentioned
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,693	8,670
Commercial, industrial and other | 7 - Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,433	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	283,378	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	19,098	15,502
Real estate - construction | 5
Financing Receivable, Recorded Investment [Line Items]
Total loans	253,407	246,806
Real estate - construction | 5W - Watch
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,292	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	2,581	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