Estimated Fair Values (Assets and Liabilities Carried at Fair Value on a Recurring Basis Significant Unobservable Inputs) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	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	$ 0		$ 1,800,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investment securities available for sale	9,019,788,000		9,019,788,000		$ 7,059,674,000
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	169,977,000	$ 149,137,000	69,685,000	$ 143,226,000
Purchases	78,000,000	8,000,000	178,595,000	11,991,000
Amounts included in net income	2,818,000	1,147,000	901,000	2,985,000
Unrealized net (losses) gains included in other comprehensive income	(81,000)	41,000	(249,000)	123,000
Transfers in	0	0	1,782,000	0
Ending balance	250,714,000	$ 157,325,000	250,714,000	$ 157,325,000
Fair Value, Measurements, Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investment securities available for sale					7,059,674,000
Fair Value, Measurements, Recurring | Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investment securities available for sale					201,566,000
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	$ 250,714,000		$ 250,714,000		69,685,000
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					$ 69,685,000