Allowance for Expected Credit Losses and Credit Quality - Credit Quality By Year (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 1,324,939
2019	5,200,853
2018	4,275,534
2017	2,707,828
2016	2,107,637
Prior	5,393,209
Revolving loans	3,419,729
Revolving loans Converted to Term	111,903
Total	24,541,632
Commercial Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,030,316
2019	4,022,223
2018	3,243,816
2017	2,016,870
2016	1,455,319
Prior	3,480,076
Revolving loans	1,892,244
Revolving loans Converted to Term	0
Total	17,140,864
Consumer and other loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	294,623
2019	1,178,630
2018	1,031,718
2017	690,958
2016	652,318
Prior	1,913,133
Revolving loans	1,527,485
Revolving loans Converted to Term	111,903
Total	7,400,768
Real estate - construction | Commercial Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	45,117
2019	336,900
2018	608,215
2017	172,612
2016	65,210
Prior	52,205
Revolving loans	42,368
Revolving loans Converted to Term	0
Total	1,322,627
Real estate - construction | Commercial Loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	45,117
2019	336,900
2018	599,002
2017	172,612
2016	65,200
Prior	51,622
Revolving loans	40,147
Revolving loans Converted to Term	0
Total	1,310,600
Real estate - construction | Commercial Loans | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	10
Prior	105
Revolving loans	2,156
Revolving loans Converted to Term	0
Total	2,271
Real estate - construction | Commercial Loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	9,213
2017	0
2016	0
Prior	475
Revolving loans	65
Revolving loans Converted to Term	0
Total	9,753
Real estate - construction | Commercial Loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	3
Revolving loans	0
Revolving loans Converted to Term	0
Total	3
Real estate - owner-occupied | Commercial Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	69,996
2019	380,116
2018	327,566
2017	365,942
2016	278,143
Prior	934,723
Revolving loans	67,653
Revolving loans Converted to Term	0
Total	2,424,139
Real estate - owner-occupied | Commercial Loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	69,996
2019	379,882
2018	319,249
2017	357,057
2016	274,226
Prior	895,032
Revolving loans	66,866
Revolving loans Converted to Term	0
Total	2,362,308
Real estate - owner-occupied | Commercial Loans | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	7,306
2017	681
2016	399
Prior	13,419
Revolving loans	400
Revolving loans Converted to Term	0
Total	22,205
Real estate - owner-occupied | Commercial Loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	234
2018	1,011
2017	8,204
2016	3,518
Prior	23,856
Revolving loans	387
Revolving loans Converted to Term	0
Total	37,210
Real estate - owner-occupied | Commercial Loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	2,416
Revolving loans	0
Revolving loans Converted to Term	0
Total	2,416
Real estate - non-owner-occupied | Commercial Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	261,614
2019	1,324,097
2018	964,005
2017	999,377
2016	883,589
Prior	1,918,111
Revolving loans	133,464
Revolving loans Converted to Term	0
Total	6,484,257
Real estate - non-owner-occupied | Commercial Loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	261,614
2019	1,323,506
2018	961,036
2017	987,814
2016	867,303
Prior	1,867,101
Revolving loans	130,929
Revolving loans Converted to Term	0
Total	6,399,303
Real estate - non-owner-occupied | Commercial Loans | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	806
2017	1,497
2016	10,248
Prior	14,561
Revolving loans	1,900
Revolving loans Converted to Term	0
Total	29,012
Real estate - non-owner-occupied | Commercial Loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	591
2018	2,059
2017	10,066
2016	6,038
Prior	31,481
Revolving loans	635
Revolving loans Converted to Term	0
Total	50,870
Real estate - non-owner-occupied | Commercial Loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	104
2017	0
2016	0
Prior	4,968
Revolving loans	0
Revolving loans Converted to Term	0
Total	5,072
Commercial and industrial | Commercial Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	653,589
2019	1,981,110
2018	1,344,030
2017	478,939
2016	228,377
Prior	575,037
Revolving loans	1,648,759
Revolving loans Converted to Term	0
Total	6,909,841
Commercial and industrial | Commercial Loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	649,228
2019	1,957,486
2018	1,307,679
2017	455,600
2016	218,485
Prior	551,933
Revolving loans	1,635,017
Revolving loans Converted to Term	0
Total	6,775,428
Commercial and industrial | Commercial Loans | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	259
2019	1,463
2018	23,811
2017	500
2016	8,093
Prior	1,436
Revolving loans	7,896
Revolving loans Converted to Term	0
Total	43,458
Commercial and industrial | Commercial Loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4,102
2019	16,838
2018	3,023
2017	20,486
2016	1,151
Prior	19,194
Revolving loans	5,846
Revolving loans Converted to Term	0
Total	70,640
Commercial and industrial | Commercial Loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	5,323
2018	9,517
2017	2,353
2016	648
Prior	2,474
Revolving loans	0
Revolving loans Converted to Term	0
Total	20,315
Residential mortgage | Consumer and other loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	245,763
2019	1,087,683
2018	892,344
2017	587,993
2016	565,932
Prior	1,469,404
Revolving loans	0
Revolving loans Converted to Term	0
Total	4,849,119
Residential mortgage | Consumer and other loans | 781 and above
Financing Receivable, Credit Quality Indicator [Line Items]
2020	45,204
2019	332,402
2018	263,099
2017	185,336
2016	153,118
Prior	387,923
Revolving loans	0
Revolving loans Converted to Term	0
Total	1,367,082
Residential mortgage | Consumer and other loans | 723-780
Financing Receivable, Credit Quality Indicator [Line Items]
2020	72,666
2019	452,401
2018	360,319
2017	233,243
2016	199,457
Prior	410,212
Revolving loans	0
Revolving loans Converted to Term	0
Total	1,728,298
Residential mortgage | Consumer and other loans | 700-722
Financing Receivable, Credit Quality Indicator [Line Items]
2020	14,430
2019	94,286
2018	93,264
2017	49,705
2016	47,736
Prior	116,470
Revolving loans	0
Revolving loans Converted to Term	0
Total	415,891
Residential mortgage | Consumer and other loans | 660-699
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,549
2019	103,361
2018	83,700
2017	37,108
2016	54,661
Prior	144,238
Revolving loans	0
Revolving loans Converted to Term	0
Total	431,617
Residential mortgage | Consumer and other loans | 620-659
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,273
2019	39,018
2018	26,883
2017	15,701
2016	15,481
Prior	94,296
Revolving loans	0
Revolving loans Converted to Term	0
Total	194,652
Residential mortgage | Consumer and other loans | 580-619
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	13,756
2018	11,717
2017	8,358
2016	11,281
Prior	51,387
Revolving loans	0
Revolving loans Converted to Term	0
Total	96,499
Residential mortgage | Consumer and other loans | 579 and below
Financing Receivable, Credit Quality Indicator [Line Items]
2020	101,641
2019	52,459
2018	53,362
2017	58,542
2016	84,198
Prior	264,878
Revolving loans	0
Revolving loans Converted to Term	0
Total	615,080
Consumer - Home equity | Consumer and other loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	13,136
2019	33,474
2018	90,369
2017	62,198
2016	61,771
Prior	240,314
Revolving loans	1,313,588
Revolving loans Converted to Term	111,903
Total	1,926,753
Consumer - Home equity | Consumer and other loans | 781 and above
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,052
2019	8,710
2018	28,297
2017	16,579
2016	15,903
Prior	56,995
Revolving loans	374,502
Revolving loans Converted to Term	21,481
Total	524,519
Consumer - Home equity | Consumer and other loans | 723-780
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,160
2019	13,680
2018	25,948
2017	19,278
2016	25,677
Prior	81,172
Revolving loans	498,082
Revolving loans Converted to Term	27,061
Total	692,058
Consumer - Home equity | Consumer and other loans | 700-722
Financing Receivable, Credit Quality Indicator [Line Items]
2020	103
2019	2,226
2018	7,915
2017	7,696
2016	5,276
Prior	18,320
Revolving loans	130,949
Revolving loans Converted to Term	13,029
Total	185,514
Consumer - Home equity | Consumer and other loans | 660-699
Financing Receivable, Credit Quality Indicator [Line Items]
2020	747
2019	3,823
2018	10,975
2017	6,434
2016	6,112
Prior	32,755
Revolving loans	163,267
Revolving loans Converted to Term	17,238
Total	241,351
Consumer - Home equity | Consumer and other loans | 620-659
Financing Receivable, Credit Quality Indicator [Line Items]
2020	351
2019	1,418
2018	1,894
2017	4,355
2016	3,630
Prior	19,419
Revolving loans	73,928
Revolving loans Converted to Term	13,822
Total	118,817
Consumer - Home equity | Consumer and other loans | 580-619
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,735
2019	875
2018	5,331
2017	2,006
2016	2,581
Prior	13,452
Revolving loans	27,526
Revolving loans Converted to Term	6,484
Total	60,990
Consumer - Home equity | Consumer and other loans | 579 and below
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5,988
2019	2,742
2018	10,009
2017	5,850
2016	2,592
Prior	18,201
Revolving loans	45,334
Revolving loans Converted to Term	12,788
Total	103,504
Consumer - Other | Consumer and other loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	35,724
2019	57,473
2018	49,005
2017	40,767
2016	24,615
Prior	203,415
Revolving loans	213,897
Revolving loans Converted to Term	0
Total	624,896
Consumer - Other | Consumer and other loans | 781 and above
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,357
2019	11,136
2018	8,566
2017	14,957
2016	5,938
Prior	22,047
Revolving loans	69,402
Revolving loans Converted to Term	0
Total	135,403
Consumer - Other | Consumer and other loans | 723-780
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5,717
2019	11,388
2018	22,356
2017	12,895
2016	5,490
Prior	32,481
Revolving loans	76,717
Revolving loans Converted to Term	0
Total	167,044
Consumer - Other | Consumer and other loans | 700-722
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,089
2019	22,224
2018	4,560
2017	3,345
2016	4,133
Prior	11,551
Revolving loans	16,713
Revolving loans Converted to Term	0
Total	63,615
Consumer - Other | Consumer and other loans | 660-699
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,803
2019	4,688
2018	5,120
2017	4,057
2016	1,113
Prior	18,128
Revolving loans	20,797
Revolving loans Converted to Term	0
Total	55,706
Consumer - Other | Consumer and other loans | 620-659
Financing Receivable, Credit Quality Indicator [Line Items]
2020	751
2019	4,245
2018	2,432
2017	1,702
2016	3,745
Prior	8,142
Revolving loans	13,553
Revolving loans Converted to Term	0
Total	34,570
Consumer - Other | Consumer and other loans | 580-619
Financing Receivable, Credit Quality Indicator [Line Items]
2020	337
2019	1,754
2018	2,689
2017	674
2016	1,611
Prior	4,922
Revolving loans	8,499
Revolving loans Converted to Term	0
Total	20,486
Consumer - Other | Consumer and other loans | 579 and below
Financing Receivable, Credit Quality Indicator [Line Items]
2020	22,670
2019	2,038
2018	3,282
2017	3,137
2016	2,585
Prior	106,144
Revolving loans	8,216
Revolving loans Converted to Term	0
Total	$ 148,072