Credit Quality - Schedule of Credit Quality Indicators Information, by Class of Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	$ 13,027,337	$ 10,356,719
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	2,996,171	2,403,437
Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	1,998,979	1,613,437
Bankcard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	10,314	14,187
Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	704,039	594,582
Owner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	1,361,629	1,049,885
Nonowner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	4,451,298	3,425,453
Construction & Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	1,504,907	1,255,738
Pass [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	2,945,266	2,348,017
Pass [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	1,848,868	1,463,797
Pass [Member] | Bankcard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	9,679	13,624
Pass [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	693,727	582,704
Pass [Member] | Owner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	1,276,088	963,503
Pass [Member] | Nonowner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	4,312,985	3,284,497
Pass [Member] | Construction & Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	1,413,706	1,126,742
Special Mention [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	18,025	18,240
Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	55,564	26,537
Special Mention [Member] | Bankcard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	449	422
Special Mention [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	9,334	10,132
Special Mention [Member] | Owner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	20,165	20,490
Special Mention [Member] | Nonowner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	57,618	36,462
Special Mention [Member] | Construction & Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	5,196	52,327
Substandard [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	32,880	36,995
Substandard [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	90,625	122,893
Substandard [Member] | Bankcard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	186	141
Substandard [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	978	1,746
Substandard [Member] | Owner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	65,376	65,892
Substandard [Member] | Nonowner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	80,695	104,494
Substandard [Member] | Construction & Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	86,005	76,669
Doubtful [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	0	185
Doubtful [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	3,922	210
Doubtful [Member] | Bankcard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	0	0
Doubtful [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	0	0
Doubtful [Member] | Owner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	0	0
Doubtful [Member] | Nonowner-Occupied [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	0	0
Doubtful [Member] | Construction & Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, credit quality indicators information	$ 0	$ 0