Allowance for Credit Losses and Credit Quality - Investment in Legacy and Acquired Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses
Total Loans	$ 22,968,295	$ 22,519,815
Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	15,628,158	15,125,322
Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Total Loans	1,219,647	1,196,366
Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	2,408,079	2,395,822
Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	6,147,864	5,796,117
Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Total Loans	5,852,568	5,737,017
Residential and Consumer Portfolio
Financing Receivable, Allowance for Credit Losses
Current	7,218,057	7,270,042
30 or more days past due	122,080	124,451
Total Loans	7,340,137	7,394,493
Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Current	4,335,779	4,290,152
30 or more days past due	79,488	69,004
Total Loans	4,415,267	4,359,156
Consumer and Other Loans | Consumer - Home equity
Financing Receivable, Allowance for Credit Losses
Current	2,184,204	2,258,659
30 or more days past due	36,444	46,035
Total Loans	2,220,648	2,304,694
Consumer and Other Loans | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	698,074	721,231
30 or more days past due	6,148	9,412
Total Loans	704,222	730,643
Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	15,270,886	14,785,998
Pass | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	1,213,202	1,182,554
Pass | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	2,332,819	2,328,999
Pass | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	6,028,301	5,687,963
Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	5,696,564	5,586,482
Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	162,729	157,229
Special Mention | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	147	1,062
Special Mention | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	33,359	25,526
Special Mention | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	82,839	78,009
Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	46,384	52,632
Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	168,776	154,402
Substandard | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	6,289	12,740
Substandard | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	41,087	41,297
Substandard | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	34,015	26,512
Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	87,385	73,853
Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	25,767	27,693
Doubtful | Commercial Loans | Commercial real estate - Construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	9	10
Doubtful | Commercial Loans | Commercial real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	814	0
Doubtful | Commercial Loans | Commercial real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	2,709	3,633
Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	$ 22,235	$ 24,050