Allowance for Credit Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current		$ 7,145,326	$ 7,055,361
Past due		101,419	81,791
Non - performing		73,799	54,057
Total		7,246,745	7,137,152
30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past due		18,211	17,992
60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past due		4,117	5,985
90 Days Past Due
Financing Receivable, Past Due [Line Items]
Past due	[1]	5,294	3,757
Commercial
Financing Receivable, Past Due [Line Items]
Total		5,620,415	5,512,796
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Current		2,901,375	3,025,505
Past due		53,614	34,087
Non - performing		46,533	25,356
Total		2,954,989	3,059,592
Commercial | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Current		1,516,639	1,466,460
Past due		7,582	14,069
Non - performing		4,505	10,911
Total		1,524,221	1,480,529
Commercial | Commercial construction
Financing Receivable, Past Due [Line Items]
Current		388,122	367,204
Past due		2,080	2,856
Non - performing		571	737
Total		390,202	370,060
Commercial | Business banking
Financing Receivable, Past Due [Line Items]
Current		1,048,241	830,735
Past due		22,223	16,055
Non - performing		11,316	9,863
Total		1,070,464	846,790
Commercial | 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past due		6,381	7,749
Commercial | 30 to 59 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past due		463	126
Commercial | 30 to 59 Days Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Past due		576	956
Commercial | 30 to 59 Days Past Due | Business banking
Financing Receivable, Past Due [Line Items]
Past due		6,758	5,093
Commercial | 60 to 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past due		0	71
Commercial | 60 to 89 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past due		0	1,589
Commercial | 60 to 89 Days Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Past due		0	1,163
Commercial | 60 to 89 Days Past Due | Business banking
Financing Receivable, Past Due [Line Items]
Past due		3,381	1,099
Commercial | 90 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past due	[1]	700	911
Commercial | 90 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past due	[1]	2,615	1,443
Commercial | 90 Days Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Past due	[1]	933	0
Commercial | 90 Days Past Due | Business banking
Financing Receivable, Past Due [Line Items]
Past due	[1]	768	0
Consumer
Financing Receivable, Past Due [Line Items]
Total		1,626,330	1,624,356
Consumer | Consumer real estate
Financing Receivable, Past Due [Line Items]
Current		1,211,865	1,283,591
Past due		15,203	11,616
Non - performing		10,616	6,063
Total		1,227,069	1,295,207
Consumer | Other consumer
Financing Receivable, Past Due [Line Items]
Current		79,083	81,866
Past due		717	3,108
Non - performing		258	1,127
Total		79,800	84,974
Consumer | 30 to 59 Days Past Due | Consumer real estate
Financing Receivable, Past Due [Line Items]
Past due		3,859	2,620
Consumer | 30 to 59 Days Past Due | Other consumer
Financing Receivable, Past Due [Line Items]
Past due		174	1,448
Consumer | 60 to 89 Days Past Due | Consumer real estate
Financing Receivable, Past Due [Line Items]
Past due		646	1,758
Consumer | 60 to 89 Days Past Due | Other consumer
Financing Receivable, Past Due [Line Items]
Past due		91	305
Consumer | 90 Days Past Due | Consumer real estate
Financing Receivable, Past Due [Line Items]
Past due	[1]	83	1,175
Consumer | 90 Days Past Due | Other consumer
Financing Receivable, Past Due [Line Items]
Past due	[1]	$ 195	$ 228

[1] Represents acquired loans that were recorded at fair value at the acquisition date and remain performing at March 31, 2020 and December 31, 2019.