Allowance for Credit Losses - Loan Balances by Year of Origination and Performing and Nonperforming Status (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
Performing
Financing Receivable, Credit Quality Indicator [Line Items]
2020	351,735
2019	1,260,585
2018	991,429
2017	694,776
2016	717,561
2015 and Prior	1,940,845
Revolving	1,192,749
Revolving-Term	23,266
Total	7,172,946
Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	300
2018	1,590
2017	1,793
2016	25,145
2015 and Prior	37,232
Revolving	6,734
Revolving-Term	1,005
Total	73,799
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 | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
2020	169,367
2019	486,935
2018	453,673
2017	371,144
2016	431,919
2015 and Prior	935,536
Revolving	59,882
Revolving-Term	0
Total	2,908,456
Commercial real estate | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	19,791
2015 and Prior	23,713
Revolving	3,008
Revolving-Term	21
Total	46,533
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 | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
2020	75,969
2019	252,166
2018	171,093
2017	108,269
2016	76,593
2015 and Prior	303,745
Revolving	531,881
Revolving-Term	0
Total	1,519,716
Commercial and industrial | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	364
2017	0
2016	709
2015 and Prior	62
Revolving	3,370
Revolving-Term	0
Total	4,505
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 | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
2020	23,004
2019	179,956
2018	117,527
2017	18,055
2016	18,986
2015 and Prior	20,020
Revolving	12,083
Revolving-Term	0
Total	389,631
Commercial construction | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	108
2016	0
2015 and Prior	463
Revolving	0
Revolving-Term	0
Total	571
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 | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
2020	40,952
2019	181,576
2018	151,933
2017	107,176
2016	94,230
2015 and Prior	355,259
Revolving	128,022
Revolving-Term	0
Total	1,059,148
Business banking | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	110
2018	1,093
2017	1,272
2016	882
2015 and Prior	7,779
Revolving	180
Revolving-Term	0
Total	11,316
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 | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
2020	33,737
2019	149,942
2018	86,401
2017	83,526
2016	90,151
2015 and Prior	318,926
Revolving	432,403
Revolving-Term	21,367
Total	1,216,453
Consumer real estate | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	190
2018	133
2017	413
2016	3,763
2015 and Prior	4,957
Revolving	176
Revolving-Term	984
Total	10,616
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 | Performing
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	7,359
Revolving	28,478
Revolving-Term	1,899
Total	79,542
Other consumer | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	258
Revolving	0
Revolving-Term	0
Total	$ 258