Allowance for Credit Losses - Loan Balances by Year of Origination and Internally Assigned Risk Rating (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 1,268,757
2019	1,240,238
2018	915,402
2017	612,707
2016	654,916
2015 and Prior	1,657,854
Revolving	1,018,345
Revolving-Term	26,649
Total	7,394,868		$ 7,137,152
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	5,810,034		5,512,796
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	277,760
2019	462,569
2018	454,551
2017	326,132
2016	425,755
2015 and Prior	821,346
Revolving	50,114
Revolving-Term	0
Total	2,818,227	$ 3,416,518	3,059,592
Commercial | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	686,461
2019	234,748
2018	156,217
2017	94,327
2016	47,706
2015 and Prior	218,559
Revolving	380,631
Revolving-Term	0
Total	1,818,649	1,720,833	1,480,529
Commercial | Consumer construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	102,950
2019	227,000
2018	83,556
2017	17,596
2016	9,854
2015 and Prior	18,686
Revolving	11,218
Revolving-Term	0
Total	470,860	375,445	370,060
Commercial | Business banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	97,471
2019	164,039
2018	137,941
2017	99,360
2016	86,333
2015 and Prior	323,499
Revolving	106,166
Revolving-Term	1,142
Total	1,015,951	0	846,790
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total			5,254,056
Commercial | Pass | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	277,760
2019	406,883
2018	446,299
2017	289,243
2016	326,634
2015 and Prior	644,888
Revolving	47,764
Revolving-Term	0
Total	2,439,471
Commercial | Pass | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	682,632
2019	209,253
2018	149,912
2017	76,118
2016	38,877
2015 and Prior	199,772
Revolving	347,762
Revolving-Term	0
Total	1,704,326
Commercial | Pass | Consumer construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	102,950
2019	223,433
2018	79,733
2017	15,844
2016	9,854
2015 and Prior	9,915
Revolving	11,127
Revolving-Term	0
Total	452,856
Commercial | Pass | Business banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	97,367
2019	163,334
2018	133,688
2017	93,886
2016	81,481
2015 and Prior	289,927
Revolving	104,721
Revolving-Term	336
Total	964,740
Commercial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total			103,878
Commercial | Special mention | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	37,197
2018	186
2017	21,850
2016	43,805
2015 and Prior	64,855
Revolving	850
Revolving-Term	0
Total	168,743
Commercial | Special mention | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,829
2019	20,396
2018	3,613
2017	968
2016	8,563
2015 and Prior	6,998
Revolving	30,552
Revolving-Term	0
Total	74,919
Commercial | Special mention | Consumer construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	3,823
2017	0
2016	0
2015 and Prior	5,033
Revolving	91
Revolving-Term	0
Total	8,947
Commercial | Special mention | Business banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	59
2018	1,049
2017	1,719
2016	1,093
2015 and Prior	7,055
Revolving	691
Revolving-Term	123
Total	11,789
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total			152,651
Commercial | Substandard | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	18,489
2018	8,066
2017	15,039
2016	54,975
2015 and Prior	105,474
Revolving	1,500
Revolving-Term	0
Total	203,543
Commercial | Substandard | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	4,861
2018	2,519
2017	14,704
2016	71
2015 and Prior	11,789
Revolving	2,317
Revolving-Term	0
Total	36,261
Commercial | Substandard | Consumer construction
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	3,567
2018	0
2017	1,752
2016	0
2015 and Prior	3,738
Revolving	0
Revolving-Term	0
Total	9,057
Commercial | Substandard | Business banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	104
2019	646
2018	3,204
2017	3,755
2016	3,759
2015 and Prior	26,517
Revolving	754
Revolving-Term	683
Total	39,422
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total			2,211
Commercial | Doubtful | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	341
2015 and Prior	6,129
Revolving	0
Revolving-Term	0
Total	6,470
Commercial | Doubtful | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	238
2018	173
2017	2,537
2016	195
2015 and Prior	0
Revolving	0
Revolving-Term	0
Total	3,143
Commercial | Doubtful | Consumer construction
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
Commercial | Doubtful | Business banking
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
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,584,834		1,624,356
Consumer | Consumer construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total	15,343		8,390
Consumer | Consumer real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	95,412
2019	136,323
2018	75,055
2017	70,907
2016	81,618
2015 and Prior	271,042
Revolving	436,604
Revolving-Term	23,519
Total	1,190,480	1,545,323	1,295,207
Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,703
2019	15,559
2018	8,082
2017	4,385
2016	3,650
2015 and Prior	4,722
Revolving	33,612
Revolving-Term	1,988
Total	80,701	$ 79,033	$ 84,974
Consumer | Pass | Consumer real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	95,412
2019	136,136
2018	74,500
2017	70,027
2016	79,784
2015 and Prior	263,696
Revolving	436,392
Revolving-Term	22,546
Total	1,178,493
Consumer | Pass | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,703
2019	15,078
2018	7,953
2017	4,270
2016	3,039
2015 and Prior	854
Revolving	33,216
Revolving-Term	641
Total	73,754
Consumer | Special mention | Consumer real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	798
2015 and Prior	153
Revolving	0
Revolving-Term	0
Total	951
Consumer | Special mention | Other consumer
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
Consumer | Substandard | Consumer real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	187
2018	555
2017	880
2016	1,036
2015 and Prior	7,193
Revolving	212
Revolving-Term	973
Total	11,036
Consumer | Substandard | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	481
2018	129
2017	115
2016	611
2015 and Prior	3,868
Revolving	396
Revolving-Term	1,347
Total	6,947
Consumer | Doubtful | Consumer real estate
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
Consumer | Doubtful | Other consumer
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