Estimated Fair Values (Assets and Liabilities Carried at Fair Value on a Recurring Basis Significant Unobservable Inputs) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	$ 1,800
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investment securities available for sale	7,789,034	$ 7,059,674
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	69,685
Amounts included in net income	(4,366)
Unrealized net (losses) gains included in other comprehensive income	(85)
Transfers in	1,782
Ending balance	67,016
Fair Value, Measurements, Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investment securities available for sale		7,059,674
Fair Value, Measurements, Recurring | Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investment securities available for sale		201,566
Significant Unobservable Inputs (Level 3 Inputs) | Fair Value, Measurements, Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investment securities available for sale	$ 67,016	69,685
Significant Unobservable Inputs (Level 3 Inputs) | Fair Value, Measurements, Recurring | Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investment securities available for sale		$ 69,685