Allowance for Credit Losses and Credit Quality - Investment in Legacy and Acquired Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses
Total Loans	$ 24,021,499	$ 22,519,815
Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	16,611,633	15,125,322
Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Total Loans	1,321,663	1,196,366
Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	2,475,326	2,395,822
Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	6,267,106	5,796,117
Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Total Loans	6,547,538	5,737,017
Residential and Consumer Portfolio
Financing Receivable, Allowance for Credit Losses
Current	7,290,371	7,270,042
30 or more days past due	119,495	124,451
Total Loans	7,409,866	7,394,493
Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Current	4,673,266	4,290,152
30 or more days past due	65,809	69,004
Total Loans	4,739,075	4,359,156
Consumer and Other Loans | Consumer - home equity
Financing Receivable, Allowance for Credit Losses
Current	1,942,091	2,258,659
30 or more days past due	45,245	46,035
Total Loans	1,987,336	2,304,694
Consumer and Other Loans | Consumer - other
Financing Receivable, Allowance for Credit Losses
Current	675,014	721,231
30 or more days past due	8,441	9,412
Total Loans	683,455	730,643
Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	16,325,326	14,785,998
Pass | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	1,300,953	1,182,554
Pass | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	2,412,953	2,328,999
Pass | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	6,185,185	5,687,963
Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	6,426,235	5,586,482
Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	127,774	157,229
Special Mention | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	10,801	1,062
Special Mention | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	19,430	25,526
Special Mention | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	45,938	78,009
Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	51,605	52,632
Sub- standard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	142,896	154,402
Sub- standard | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	9,909	12,740
Sub- standard | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	40,570	41,297
Sub- standard | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	35,726	26,512
Sub- standard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	56,691	73,853
Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	15,637	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	2,373	0
Doubtful | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	257	3,633
Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	$ 13,007	$ 24,050