Allowance for Credit Losses - Allowance for Credit Loss Activity (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 114,609		$ 61,479	$ 62,224	$ 60,996	$ 60,996
Provision for credit losses on loans	19,302		4,913	123,564	12,767
Charge-offs	(13,667)		(4,700)	(93,416)	(13,391)
Recoveries	754		423	1,274	1,743
Net (Charge-offs)/Recoveries	(12,913)		(4,277)	(92,142)	(11,648)
Balance at End of Period	120,998	$ 114,609	62,115	$ 120,998	62,115	$ 62,224	$ 60,996
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Customer fraud, check kiting
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Charge-offs		(58,700)
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	57,730		32,836	$ 30,577	33,707	$ 33,707
Provision for credit losses on loans	23,839		1,292	52,185	822
Charge-offs	(10,187)		(2,304)	(16,229)	(2,833)
Recoveries	172		6	211	134
Net (Charge-offs)/Recoveries	(10,015)		(2,298)	(16,018)	(2,699)
Balance at End of Period	71,554	57,730	31,830	71,554	31,830	30,577	$ 33,707
Commercial | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	19,164		13,227	15,681	11,596	11,596
Provision for credit losses on loans	(4,155)		2,397	62,949	10,432
Charge-offs	(1,196)		(1,467)	(72,692)	(8,379)
Recoveries	398		210	420	718
Net (Charge-offs)/Recoveries	(798)		(1,257)	(72,272)	(7,661)
Balance at End of Period	14,211	19,164	14,367	14,211	14,367	15,681	11,596
Commercial | Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	8,874		7,254	7,900	7,983	7,983
Provision for credit losses on loans	(1,617)		620	2,712	(112)
Charge-offs	0		0	0	0
Recoveries	1		1	22	4
Net (Charge-offs)/Recoveries	1		1	22	4
Balance at End of Period	7,258	8,874	7,875	7,258	7,875	7,900	7,983
Commercial | Business banking
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	14,404			0
Provision for credit losses on loans	2,072			4,197
Charge-offs	(1,748)			(2,469)
Recoveries	64			166
Net (Charge-offs)/Recoveries	(1,684)			(2,303)
Balance at End of Period	14,792	14,404		14,792		0
Consumer | Consumer real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	11,585		6,571	6,337	6,187	6,187
Provision for credit losses on loans	(797)		65	32	367
Charge-offs	(252)		(404)	(470)	(815)
Recoveries	41		102	153	595
Net (Charge-offs)/Recoveries	(211)		(302)	(317)	(220)
Balance at End of Period	10,577	11,585	6,334	10,577	6,334	6,337	6,187
Consumer | Other consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	2,852		1,591	1,729	1,523	1,523
Provision for credit losses on loans	(40)		539	1,489	1,258
Charge-offs	(284)		(525)	(1,556)	(1,364)
Recoveries	78		104	302	292
Net (Charge-offs)/Recoveries	(206)		(421)	(1,254)	(1,072)
Balance at End of Period	$ 2,606	$ 2,852	$ 1,709	2,606	$ 1,709	1,729	$ 1,523
Impact of ASU 2016-13 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				27,352
Balance at End of Period						27,352
Impact of ASU 2016-13 Adoption | Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				4,810
Balance at End of Period						4,810
Impact of ASU 2016-13 Adoption | Commercial | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				7,853
Balance at End of Period						7,853
Impact of ASU 2016-13 Adoption | Commercial | Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				(3,376)
Balance at End of Period						(3,376)
Impact of ASU 2016-13 Adoption | Commercial | Business banking
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				12,898
Balance at End of Period						12,898
Impact of ASU 2016-13 Adoption | Consumer | Consumer real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				4,525
Balance at End of Period						4,525
Impact of ASU 2016-13 Adoption | Consumer | Other consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 642
Balance at End of Period						$ 642