Summary of Significant Accounting Policies Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other assets			$ 589,420			$ 516,538
Accrued expenses and other liabilities	[1]		470,073			409,787
Transfer of securities from held to maturity to available for sale			692,000
Operating lease right-of-use asset			48,127
Operating lease liability			$ 52,011
Furniture and Fixtures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Useful life			12 years	7 years
Effect of change on net income			$ 230
Effect of Change | Change in Accounting Policy for Netting Agreements
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other assets						(33,000)
Accrued expenses and other liabilities						$ (33,000)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset					$ 52,000
Operating lease liability					$ 56,000
Accounting Standards Update 2019-04
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transfer of securities from held to maturity to available for sale			$ 692,000
Forecast | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated reduction to tangible common equity		(0.25%)
Reasonable and Supportable Period2016-13 [Text Block]		1 year
Straight Line Reversion to Historical Losses 2016-13 [Text Block]		1 year
Minimum | Forecast | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percent increase in allowance for credit losses		30.00%
Maximum | Forecast | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percent increase in allowance for credit losses		40.00%

[1] (b) 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.