Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment	$ 2,610,244	$ 2,538,339	$ 2,507,611		$ 4,313,915		$ 4,289,516	$ 4,228,507	$ 4,112,326	$ 4,056,442
Allowance for credit losses on loans and leases	345,049	356,745		$ 207,628	157,629		156,288	151,069		144,871
Reserve for Unfunded Commitments	24,306	26,368		8,344	5,106		5,085	4,857		4,523
$ Increase (decrease)	2,610,244	2,538,339	2,507,611		4,313,915		4,289,516	4,228,507	4,112,326	4,056,442
Commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	134,055	152,828		55,924	50,847		51,201	48,997		47,904
Reserve for Unfunded Commitments	20,428	20,808		4,564	534		604	561		628
Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	164,836	152,615		117,829	73,820		70,956	68,353		63,957
Reserve for Unfunded Commitments	1,723	2,921		2,052	2,539		2,784	2,595		2,250
Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	35,194	40,548		26,813	24,714		24,556	23,654		22,034
Reserve for Unfunded Commitments	1,519	2,061		1,416	149		102	157		160
Consumer & Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	10,964	10,754		7,062	8,248		9,575	10,065		10,976
Reserve for Unfunded Commitments	636	578		312	1,884		1,595	1,544		1,485
Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases				207,600	49,999
Reserve for Unfunded Commitments				8,300
Accounting Standards Update 2016-13 [Member] | Commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases					5,077
Accounting Standards Update 2016-13 [Member] | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases					44,009
Accounting Standards Update 2016-13 [Member] | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases					2,099
Accounting Standards Update 2016-13 [Member] | Consumer & Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases					(1,186)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment	(1,036,931)	(1,115,414)	(1,168,340)		770,366		733,059	695,003	629,877	602,482
$ Increase (decrease)	$ (1,036,931)	$ (1,115,414)	$ (1,168,340)		770,366		$ 733,059	$ 695,003	$ 629,877	602,482
Cumulative Effect, Period of Adoption, Adjusted Balance | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases				207,628
Reserve for Unfunded Commitments				8,344
Cumulative Effect, Period of Adoption, Adjusted Balance | Accounting Standards Update 2016-13 [Member] | Commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases				55,924
Reserve for Unfunded Commitments				4,564
Cumulative Effect, Period of Adoption, Adjusted Balance | Accounting Standards Update 2016-13 [Member] | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases				117,829
Reserve for Unfunded Commitments				2,052
Cumulative Effect, Period of Adoption, Adjusted Balance | Accounting Standards Update 2016-13 [Member] | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases				26,813
Reserve for Unfunded Commitments				1,416
Cumulative Effect, Period of Adoption, Adjusted Balance | Accounting Standards Update 2016-13 [Member] | Consumer & Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases				7,062
Reserve for Unfunded Commitments				$ 312
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment					(40,181)	[1]				(244)	[2]
$ Increase (decrease)					(40,181)	[1]				(244)	[2]
% Increase (decrease)				33.00%
Financing Receivable, Allowance for Credit Loss and Off-Balance Sheet, Credit Loss, Liability				$ 53,237
Cumulative Effect, Period of Adoption, Adjustment | Commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
% Increase (decrease)				18.00%
Financing Receivable, Allowance for Credit Loss and Off-Balance Sheet, Credit Loss, Liability				$ 9,107
Cumulative Effect, Period of Adoption, Adjustment | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
% Increase (decrease)				57.00%
Financing Receivable, Allowance for Credit Loss and Off-Balance Sheet, Credit Loss, Liability				$ 43,522
Cumulative Effect, Period of Adoption, Adjustment | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
% Increase (decrease)				14.00%
Financing Receivable, Allowance for Credit Loss and Off-Balance Sheet, Credit Loss, Liability				$ 3,366
Cumulative Effect, Period of Adoption, Adjustment | Consumer & Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
% Increase (decrease)				(27.00%)
Financing Receivable, Allowance for Credit Loss and Off-Balance Sheet, Credit Loss, Liability				$ (2,758)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment				(40,200)	(40,181)	[1]				(244)	[2]
$ Increase (decrease)				$ (40,200)	$ (40,181)	[1]				$ (244)	[2]

[1] The cumulative effect adjustment relates to the implementation of new accounting guidance for the allowance for credit losses on January 1, 2020. Refer to Note 1 for discussion of the new accounting guidance.

[2] The cumulative effect adjustment relates to the implementation of new accounting guidance for leases on January 1, 2019.