Basis of Presentation and Accounting Policies (Narrative) (Details) numberOfIndividuals in Thousands, numberOfCustomers in Thousands, $ in Thousands		6 Months Ended
	Jan. 01, 2020 USD ($)	Jun. 30, 2020 USD ($) branch ATM numberOfIndividuals numberOfCustomers	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)		Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Number of branches | branch		293
Number of ATMs | ATM		389
PPP funded amount		$ 2,900,000
PPP number of customers | numberOfCustomers		19
PPP average loan amount		$ 150
PPP individuals employed by customers | numberOfIndividuals		335
Life of loans		1 year
Revert period		1 year
Basis Of Presentation [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Stockholders' equity		$ 5,052,968	$ 4,753,816	$ 5,065,205	$ 4,941,690		$ 4,597,753	$ 3,133,602
Impact from Adoption
Basis Of Presentation [Line Items]
Stockholders' equity					$ (35,721)	[1]		$ 4,270
Unfunded Loan Commitment
Basis Of Presentation [Line Items]
Cost basis of loans	$ 62,200
Unfunded Loan Commitment | Impact from Adoption
Basis Of Presentation [Line Items]
Stockholders' equity	$ 35,700

[1] For additional information, see "Part I - Item 1. Financial Statements and Supplementary Data - Note 1 - Basis of Presentation" in this Report.