Allowance for Credit Losses and Credit Quality - Investment in Legacy and Acquired Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses
Total Loans	$ 23,676,537	$ 22,519,815
Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	16,299,881	15,125,322
Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Total Loans	1,330,014	1,196,366
Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	2,468,061	2,395,822
Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Total Loans	6,011,681	5,796,117
Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Total Loans	6,490,125	5,737,017
Residential and Consumer Portfolio
Financing Receivable, Allowance for Credit Losses
Current	7,245,248	7,270,042
30 or more days past due	131,408	124,451
Total Loans	7,376,656	7,394,493
Residential mortgage loans | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Current	4,567,633	4,290,152
30 or more days past due	82,112	69,004
Total Loans	4,649,745	4,359,156
Consumer and Other Loans | Consumer - Home equity
Financing Receivable, Allowance for Credit Losses
Current	2,011,913	2,258,659
30 or more days past due	41,675	46,035
Total Loans	2,053,588	2,304,694
Consumer and Other Loans | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	665,702	721,231
30 or more days past due	7,621	9,412
Total Loans	673,323	730,643
Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	15,977,343	14,785,998
Pass | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	1,300,847	1,182,554
Pass | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	2,409,221	2,328,999
Pass | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	5,926,537	5,687,963
Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	6,340,738	5,586,482
Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	158,342	157,229
Special Mention | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	19,631	1,062
Special Mention | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	19,786	25,526
Special Mention | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	47,494	78,009
Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	71,431	52,632
Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	148,246	154,402
Substandard | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	9,536	12,740
Substandard | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	38,214	41,297
Substandard | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	37,193	26,512
Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	63,303	73,853
Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	15,940	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	840	0
Doubtful | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	447	3,633
Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	14,653	$ 24,050
Loss | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	0
Loss | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	10
Loss | Commercial Loans | Real estate - construction
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	0
Loss | Commercial Loans | Real estate - owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	0
Loss | Commercial Loans | Real estate - non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Gross loans and leases receivable	$ 10