Allowance for Credit Losses and Credit Quality - Investment in Legacy and Acquired Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	$ 22,600	$ 0
Acquired Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	2,208,758	2,370,047
Acquired Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(30,855)	(38,153)
Total Loans	1,415,846	1,529,182
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	11,001	8,759
Total Loans	61,956	61,408
Acquired Loans | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(39,982)	(42,419)
Total Loans	1,037,409	1,117,544
Acquired Loans | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(1,869)	(4,487)
Total Loans	315,373	348,326
Acquired Loans | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(5)	(6)
Total Loans	1,108	1,904
Acquired Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(49,974)	(53,753)
Current	809,710	859,474
30 or more days past due	33,176	35,144
Total Loans	792,912	840,865
Acquired Loans | Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(29,045)	(32,030)
Current	404,276	424,300
30 or more days past due	19,268	20,914
Total Loans	394,499	413,184
Acquired Loans | Consumer and Other Loans | Consumer - Home equity
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(16,134)	(17,323)
Current	352,836	377,021
30 or more days past due	12,971	12,807
Total Loans	349,673	372,505
Acquired Loans | Consumer and Other Loans | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	0	(8)
Current	26	12
30 or more days past due	0	0
Total Loans	26	4
Acquired Loans | Consumer and Other Loans | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Total Loans	501	468
Acquired Loans | Consumer and Other Loans | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(4,795)	(4,392)
Current	52,572	58,141
30 or more days past due	937	1,423
Total Loans	48,714	55,172
Acquired Loans | Consumer and Other Loans | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Total Loans	48,213	54,704
Acquired Loans | Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	1,353,764	1,461,625
Acquired Loans | Pass | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	45,192	46,498
Acquired Loans | Pass | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	1,014,057	1,090,063
Acquired Loans | Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	293,402	323,154
Acquired Loans | Pass | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	1,113	1,910
Acquired Loans | Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	21,287	21,159
Acquired Loans | Special Mention | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	472	459
Acquired Loans | Special Mention | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	15,408	19,284
Acquired Loans | Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	5,407	1,416
Acquired Loans | Special Mention | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	0	0
Acquired Loans | Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	67,198	80,003
Acquired Loans | Substandard | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	2,717	3,118
Acquired Loans | Substandard | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	46,451	49,136
Acquired Loans | Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	18,030	27,749
Acquired Loans | Substandard | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	0	0
Acquired Loans | Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	4,452	4,525
Acquired Loans | Doubtful | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	2,574	2,574
Acquired Loans | Doubtful | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	1,475	1,457
Acquired Loans | Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	403	494
Acquired Loans | Doubtful | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	0	0
Acquired Loans | Loss | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross		23
Acquired Loans | Loss | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross		0
Acquired Loans | Loss | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross		23
Acquired Loans | Loss | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross		0
Acquired Loans | Loss | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross		0
Legacy Loans
Financing Receivable, Allowance for Credit Losses
Current	12,702,980	12,447,375
Total Loans	12,923,444	12,694,924
Legacy Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	9,581,229	9,377,399
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Current	880,721	740,761
Total Loans	882,339	740,761
Legacy Loans | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Current	4,973,144	4,863,902
Total Loans	4,996,170	4,882,553
Legacy Loans | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Current	3,102,314	3,158,700
Total Loans	3,140,205	3,194,796
Legacy Loans | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Current	447,128	407,434
Total Loans	562,515	559,289
Legacy Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Current	3,299,673	3,276,578
30 or more days past due	42,542	40,947
Total Loans	3,342,215	3,317,525
Legacy Loans | Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Current	884,571	836,509
30 or more days past due	17,288	17,707
Total Loans	901,859	854,216
Legacy Loans | Consumer and Other Loans | Consumer - Home equity
Financing Receivable, Allowance for Credit Losses
Current	1,779,409	1,768,763
30 or more days past due	17,714	14,658
Total Loans	1,797,123	1,783,421
Legacy Loans | Consumer and Other Loans | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Current	107,519	127,054
30 or more days past due	2,655	3,994
Total Loans	110,174	131,048
Legacy Loans | Consumer and Other Loans | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Current	82,636	81,602
30 or more days past due	976	922
Total Loans	83,612	82,524
Legacy Loans | Consumer and Other Loans | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	445,538	462,650
30 or more days past due	3,909	3,666
Total Loans	449,447	466,316
Legacy Loans | Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	9,138,375	8,890,604
Legacy Loans | Pass | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	876,872	734,687
Legacy Loans | Pass | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	4,882,222	4,801,494
Legacy Loans | Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	3,064,306	3,112,300
Legacy Loans | Pass | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	314,975	242,123
Legacy Loans | Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	150,286	138,209
Legacy Loans | Special Mention | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	1,119	2,203
Legacy Loans | Special Mention | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	52,083	26,159
Legacy Loans | Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	31,615	29,763
Legacy Loans | Special Mention | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	65,469	80,084
Legacy Loans | Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	246,413	319,694
Legacy Loans | Substandard | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	4,348	3,871
Legacy Loans | Substandard | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	61,865	54,900
Legacy Loans | Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	27,014	35,199
Legacy Loans | Substandard | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	153,186	225,724
Legacy Loans | Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	46,155	28,892
Legacy Loans | Doubtful | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	0	0
Legacy Loans | Doubtful | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	0	0
Legacy Loans | Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	17,270	17,534
Legacy Loans | Doubtful | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable before Fees, Gross	$ 28,885	$ 11,358