Allowance for Credit Losses and Credit Quality - Investment in Legacy and Acquired Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses
Total Loans	$ 23,355,311	$ 22,519,815
Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	15,980,029	15,125,322
Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Total Loans	1,342,984	1,196,366
Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	2,373,143	2,395,822
Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	6,102,143	5,796,117
Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Total Loans	6,161,759	5,737,017
Residential and Consumer Portfolio
Financing Receivable, Allowance for Credit Losses
Current	7,255,803	7,270,042
30 or more days past due	119,479	124,451
Total Loans	7,375,282	7,394,493
Residential mortgage loans | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Current	4,459,706	4,290,152
30 or more days past due	78,488	69,004
Total Loans	4,538,194	4,359,156
Consumer and Other Loans | Consumer - Home equity
Financing Receivable, Allowance for Credit Losses
Current	2,113,237	2,258,659
30 or more days past due	34,660	46,035
Total Loans	2,147,897	2,304,694
Consumer and Other Loans | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	682,860	721,231
30 or more days past due	6,331	9,412
Total Loans	689,191	730,643
Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	15,658,849	14,785,998
Pass | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	1,320,007	1,182,554
Pass | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	2,308,176	2,328,999
Pass | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	6,011,800	5,687,963
Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	6,018,866	5,586,482
Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	138,774	157,229
Special Mention | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	12,262	1,062
Special Mention | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	25,114	25,526
Special Mention | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	48,323	78,009
Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	53,075	52,632
Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	161,340	154,402
Substandard | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	10,715	12,740
Substandard | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	37,126	41,297
Substandard | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	39,832	26,512
Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	73,667	73,853
Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	21,066	27,693
Doubtful | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	0	10
Doubtful | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	2,727	0
Doubtful | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	2,188	3,633
Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	$ 16,151	$ 24,050