Estimated Fair Values (Assets and Liabilities Carried at Fair Value on a Recurring Basis Significant Unobservable Inputs) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	8,619,282,000		8,619,282,000		$ 7,059,674,000
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	67,016,000	$ 146,287,000	69,685,000	$ 143,226,000
Purchases	100,595,000	991,000	100,595,000	3,991,000
Amounts included in net income	2,449,000	1,822,000	(1,917,000)	1,838,000
Unrealized net (losses) gains included in other comprehensive income	(83,000)	37,000	(168,000)	82,000
Transfers in	0	0	1,782,000	0
Ending balance	169,977,000	$ 149,137,000	169,977,000	$ 149,137,000
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	$ 169,977,000		$ 169,977,000		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