Loans and Allowance for Credit Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	$ 192,460	$ 206,569
Past Due 90 Days or More and Accruing	560	1,353
Nonaccrual	152,881	157,484
Current	11,876,517	12,037,323
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	51,992	54,596
Past Due 90 Days or More and Accruing	49	258
Nonaccrual	25,791	24,954
Current	2,191,893	2,193,808
Consumer Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	6,017	5,937
Past Due 90 Days or More and Accruing	158	456
Nonaccrual	4,763	3,087
Current	547,247	583,385
Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	8,844	11,966
Past Due 90 Days or More and Accruing	89	377
Nonaccrual	3,298	2,595
Current	1,025,503	1,047,667
Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	1,655	2,690
Past Due 90 Days or More and Accruing	26	170
Nonaccrual	1,025	1,599
Current	169,416	152,022
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	36,340	42,606
Past Due 90 Days or More and Accruing	98	52
Nonaccrual	34,046	38,648
Current	3,006,450	3,190,364
Commercial Real Estate [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	14,508	13,824
Nonaccrual	14,508	13,824
Current	538,317	490,801
Commercial Real Estate [Member] | Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	73,104	74,950
Past Due 90 Days or More and Accruing	140	40
Nonaccrual	69,450	72,777
Current	4,397,691	4,379,276
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	34,208	40,443
30 to 59 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	25,297	25,947
30 to 59 Days Past Due [Member] | Consumer Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	786	1,434
30 to 59 Days Past Due [Member] | Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	4,667	7,091
30 to 59 Days Past Due [Member] | Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	494	711
30 to 59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	1,308	3,627
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	1,656	1,633
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	4,811	7,289
60 to 89 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	855	3,437
60 to 89 Days Past Due [Member] | Consumer Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	310	960
60 to 89 Days Past Due [Member] | Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	790	1,903
60 to 89 Days Past Due [Member] | Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	110	210
60 to 89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	888	279
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	$ 1,858	$ 500