Loans and Allowance for Credit Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	$ 164,469	$ 206,569
Past Due 90 Days or More and Accruing	703	1,353
Nonaccrual	138,498	157,484
Current	11,853,179	12,037,323
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	32,989	54,596
Past Due 90 Days or More and Accruing	64	258
Nonaccrual	20,759	24,954
Current	2,203,634	2,193,808
Consumer Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	6,135	5,937
Past Due 90 Days or More and Accruing	233	456
Nonaccrual	4,776	3,087
Current	549,770	583,385
Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	10,472	11,966
Past Due 90 Days or More and Accruing	190	377
Nonaccrual	3,444	2,595
Current	995,063	1,047,667
Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	2,112	2,690
Past Due 90 Days or More and Accruing	116	170
Nonaccrual	1,196	1,599
Current	154,791	152,022
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	41,379	42,606
Past Due 90 Days or More and Accruing	100	52
Nonaccrual	37,880	38,648
Current	2,909,180	3,190,364
Commercial Real Estate [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	13,955	13,824
Nonaccrual	13,655	13,824
Current	578,393	490,801
Commercial Real Estate [Member] | Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	57,427	74,950
Past Due 90 Days or More and Accruing		40
Nonaccrual	56,788	72,777
Current	4,462,348	4,379,276
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	19,159	40,443
30 to 59 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	8,620	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	1,008	1,434
30 to 59 Days Past Due [Member] | Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	6,069	7,091
30 to 59 Days Past Due [Member] | Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	695	711
30 to 59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	2,218	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	549	1,633
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	6,109	7,289
60 to 89 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	3,546	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	118	960
60 to 89 Days Past Due [Member] | Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	769	1,903
60 to 89 Days Past Due [Member] | Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	105	210
60 to 89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	1,181	279
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivables, Past Due	300
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	$ 90	$ 500