Allowance for Credit Losses - Loan Balances by Year of Origination and Internally Assigned Risk Rating (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 351,735
2019	1,260,885
2018	993,019
2017	696,569
2016	742,706
2015 and Prior	1,978,077
Revolving	1,199,483
Revolving-Term	24,271
Total	7,246,745	$ 7,137,152
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	169,367
2019	486,935
2018	453,673
2017	371,144
2016	451,710
2015 and Prior	959,249
Revolving	62,890
Revolving-Term	21
Total	2,954,989
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	169,367
2019	483,699
2018	452,024
2017	359,553
2016	416,420
2015 and Prior	882,326
Revolving	59,032
Revolving-Term	0
Total	2,822,421
Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	3,236
2018	187
2017	11,591
2016	10,005
2015 and Prior	26,575
Revolving	850
Revolving-Term	0
Total	52,444
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	1,462
2017	0
2016	25,285
2015 and Prior	50,348
Revolving	3,008
Revolving-Term	21
Total	80,124
Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	75,969
2019	252,166
2018	171,457
2017	108,269
2016	77,302
2015 and Prior	303,807
Revolving	535,251
Revolving-Term	0
Total	1,524,221
Commercial and industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	75,969
2019	240,261
2018	165,631
2017	103,537
2016	73,798
2015 and Prior	282,138
Revolving	486,045
Revolving-Term	0
Total	1,427,379
Commercial and industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	6,239
2018	3,525
2017	947
2016	1,883
2015 and Prior	9,437
Revolving	31,890
Revolving-Term	0
Total	53,921
Commercial and industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	5,666
2018	2,301
2017	3,785
2016	1,621
2015 and Prior	12,232
Revolving	17,316
Revolving-Term	0
Total	42,921
Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	23,004
2019	179,956
2018	117,527
2017	18,163
2016	18,986
2015 and Prior	20,483
Revolving	12,083
Revolving-Term	0
Total	390,202
Commercial construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	23,004
2019	179,956
2018	117,527
2017	17,122
2016	18,986
2015 and Prior	11,586
Revolving	11,992
Revolving-Term	0
Total	380,173
Commercial construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	5,117
Revolving	91
Revolving-Term	0
Total	5,208
Commercial construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	1,041
2016	0
2015 and Prior	3,780
Revolving	0
Revolving-Term	0
Total	4,821
Business banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	40,952
2019	181,686
2018	153,026
2017	108,448
2016	95,112
2015 and Prior	363,038
Revolving	128,202
Revolving-Term	0
Total	1,070,464
Business banking | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	40,952
2019	181,242
2018	148,419
2017	103,027
2016	89,925
2015 and Prior	327,333
Revolving	125,800
Revolving-Term	0
Total	1,016,698
Business banking | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	62
2018	1,496
2017	1,775
2016	1,181
2015 and Prior	7,902
Revolving	737
Revolving-Term	0
Total	13,153
Business banking | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	382
2018	3,111
2017	3,646
2016	4,006
2015 and Prior	27,803
Revolving	1,665
Revolving-Term	0
Total	40,613
Consumer real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	33,737
2019	150,132
2018	86,534
2017	83,939
2016	93,914
2015 and Prior	323,883
Revolving	432,579
Revolving-Term	22,351
Total	1,227,069
Consumer real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	33,737
2019	149,942
2018	86,463
2017	83,402
2016	92,013
2015 and Prior	316,169
Revolving	432,274
Revolving-Term	20,966
Total	1,214,966
Consumer real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	798
2015 and Prior	304
Revolving	0
Revolving-Term	0
Total	1,102
Consumer real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	190
2018	71
2017	537
2016	1,103
2015 and Prior	7,410
Revolving	305
Revolving-Term	1,385
Total	11,001
Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,706
2019	10,010
2018	10,802
2017	6,606
2016	5,682
2015 and Prior	7,617
Revolving	28,478
Revolving-Term	1,899
Total	79,800
Other consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,706
2019	9,522
2018	10,670
2017	6,474
2016	5,088
2015 and Prior	3,365
Revolving	28,156
Revolving-Term	773
Total	72,754
Other consumer | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	0
Revolving	0
Revolving-Term	0
Total	0
Other consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	488
2018	132
2017	132
2016	594
2015 and Prior	4,252
Revolving	322
Revolving-Term	1,126
Total	$ 7,046