Loans - Risk Category of Loans by Class of Loans, Excluding Purchased Credit Impaired Loans (Detail) $ in Thousands	Dec. 31, 2019 USD ($)
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 11,548,688
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	191,249
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	104,019
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	2,512,544
Residential Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	2,455,752
Residential Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	31,189
Residential Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	25,603
Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	6,151,309
Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	118,412
Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	55,387
Land, Development, Construction [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	989,860
Land, Development, Construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	6,418
Land, Development, Construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	3,645
Commercial and Industrial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	1,705,862
Commercial and Industrial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	35,070
Commercial and Industrial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	18,142
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	247,307
Consumer and Other [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	245,905
Consumer and Other [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	160
Consumer and Other [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 1,242