Loans and Allowance for Credit Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	$ 206,569	$ 166,934
Past Due 90 Days or More and Accruing	1,353	894
Nonaccrual	157,484	124,927
Current	12,037,323	10,951,187
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	54,596	44,535
Past Due 90 Days or More and Accruing	258
Nonaccrual	24,954	22,171
Current	2,193,808	2,122,518
Consumer Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	5,937	6,950
Past Due 90 Days or More and Accruing	456	68
Nonaccrual	3,087	5,185
Current	583,385	500,557
Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	11,966	11,116
Past Due 90 Days or More and Accruing	377	532
Nonaccrual	2,595	2,695
Current	1,047,667	1,137,556
Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	2,690	10,880
Past Due 90 Days or More and Accruing	170	150
Nonaccrual	1,599	5,249
Current	152,022	212,188
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	42,606	28,692
Past Due 90 Days or More and Accruing	52	144
Nonaccrual	38,648	27,202
Current	3,190,364	2,688,577
Commercial Real Estate [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	13,824	2,110
Nonaccrual	13,824	2,110
Current	490,801	372,196
Commercial Real Estate [Member] | Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	74,950	62,651
Past Due 90 Days or More and Accruing	40
Nonaccrual	72,777	60,315
Current	4,379,276	3,917,595
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	40,443	34,102
30 to 59 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	25,947	18,948
30 to 59 Days Past Due [Member] | Consumer Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	1,434	1,467
30 to 59 Days Past Due [Member] | Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	7,091	6,487
30 to 59 Days Past Due [Member] | Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	711	3,967
30 to 59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	3,627	986
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] | Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	1,633	2,247
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	7,289	7,011
60 to 89 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	3,437	3,416
60 to 89 Days Past Due [Member] | Consumer Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	960	230
60 to 89 Days Past Due [Member] | Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	1,903	1,402
60 to 89 Days Past Due [Member] | Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	210	1,514
60 to 89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	279	360
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member] | Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	$ 500	$ 89