Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands			3 Months Ended			9 Months Ended	12 Months Ended
		Jan. 01, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets	[1]			$ (597,242)			$ (597,242)	$ (516,538)
Accrued expenses and other liabilities(a)				489,868			489,868	409,787
Transfer of held to maturity securities to available for sale securities (adoption of ASU 2019-04)					$ 692,000		692,414	0	$ 0
Operating Lease, Right-of-Use Asset				45,381			45,381
Present value of lease payments				$ 49,292			49,292
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification								(13,738)
Furniture, fixtures and other equipment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life				12 years		7 years
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income							$ 915
Restatement Adjustment [Member] | Change In Accounting Policy For Netting Agreements [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets								(33,000)
Accrued expenses and other liabilities(a)								$ (33,000)
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset										$ 52,000
Present value of lease payments										$ 56,000
Accounting Standards Update 2019-04 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transfer of held to maturity securities to available for sale securities (adoption of ASU 2019-04)					$ 692,000
Forecast [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reasonable and Supportable Period 2016-13			2 years
Core Deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life							10 years
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage							0.00%
Minimum [Member] | Furniture, fixtures and other equipment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life							3 years
Minimum [Member] | Subsequent Event [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		$ 70,000
Minimum [Member] | Forecast [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact To Tangible Common Equity, Percent Increase (Decrease)			(0.21%)
Maximum
Finite-Lived Intangible Assets [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage							150.00%
Maximum | Furniture, fixtures and other equipment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life							15 years
Maximum | Subsequent Event [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		$ 80,000
Maximum | Forecast [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact To Tangible Common Equity, Percent Increase (Decrease)			(0.24%)

[1] (a) During the third quarter of 2019, the Corporation made a change in accounting policy to offset derivative assets and liabilities and cash collateral with the same counterparty where it has a legally enforceable master netting agreement in place. The change had no impact on either earnings or equity. The Corporation believes that this change is a preferable method of accounting as it provides a better reflection of the assets and liabilities on the face of the consolidated balance sheets. Adoption of this change was voluntary and has been adopted retrospectively with all prior periods presented herein revised.