Allowance for Credit Losses and Credit Quality - Investment in Legacy and Acquired Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses
Total Loans	$ 21,706,090	$ 20,078,181
Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	14,574,633	14,073,297
Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Total Loans	1,199,625	1,240,396
Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	2,612,244	2,529,885
Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	5,437,082	5,167,949
Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Total Loans	5,325,682	5,135,067
Residential and Consumer Portfolio
Financing Receivable, Allowance for Credit Losses
Current	6,966,104	5,857,746
30 or more days past due	165,353	147,138
Total Loans	7,131,457	6,004,884
Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Current	3,859,751	2,962,043
30 or more days past due	111,316	94,309
Total Loans	3,971,067	3,056,352
Consumer and Other Loans | Consumer - Home equity
Financing Receivable, Allowance for Credit Losses
Current	2,376,171	2,250,205
30 or more days past due	45,015	42,070
Total Loans	2,421,186	2,292,275
Consumer and Other Loans | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Current	48,587	59,836
30 or more days past due	2,084	2,857
Total Loans	50,671	62,693
Consumer and Other Loans | Consumer - Credit card
Financing Receivable, Allowance for Credit Losses
Current	92,497	95,263
30 or more days past due	764	1,105
Total Loans	93,261	96,368
Consumer and Other Loans | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	589,098	490,399
30 or more days past due	6,174	6,797
Total Loans	595,272	497,196
Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	14,078,168	13,564,833
Pass | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	1,153,604	1,189,490
Pass | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	2,464,072	2,388,715
Pass | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	5,335,816	5,104,074
Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	5,124,676	4,882,554
Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	200,701	209,925
Special Mention | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	16,399	20,351
Special Mention | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	83,356	82,114
Special Mention | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	45,905	19,311
Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	55,041	88,149
Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	244,234	258,794
Substandard | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	29,610	30,541
Substandard | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	56,409	56,590
Substandard | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	51,089	42,702
Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	107,126	128,961
Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	51,530	39,627
Doubtful | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	12	14
Doubtful | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	8,407	2,466
Doubtful | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	4,272	1,744
Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	$ 38,839	35,403
Loss | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable		118
Loss | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable		0
Loss | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable		0
Loss | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable		118
Loss | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable		$ 0