Securities - Schedule of Securities with Gross Unrealized Losses, Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
(In thousands)
Less than 12 months, fair value	$ 10,178,000	$ 13,509,000
Less than 12 months, unrealized (losses)	(86,000)	(56,000)
12 months or more, fair value	0	5,651,000
12 months or more, unrealized (losses)	0	(44,000)
Total, fair value	10,178,000	19,160,000
Total, unrealized (losses)	(86,000)	(100,000)
Obligations of U.S. Government Corporations and Agencies [Member]
(In thousands)
Less than 12 months, fair value	6,097,000	11,460,000
Less than 12 months, unrealized (losses)	(16,000)	(42,000)
12 months or more, fair value	0	5,651,000
12 months or more, unrealized (losses)	0	(44,000)
Total, fair value	6,097,000	17,111,000
Total, unrealized (losses)	(16,000)	(86,000)
Obligations of States and Political Subdivisions [Member]
(In thousands)
Less than 12 months, fair value	4,081,000	2,049,000
Less than 12 months, unrealized (losses)	(70,000)	(14,000)
12 months or more, fair value	0	0
12 months or more, unrealized (losses)	0	0
Total, fair value	4,081,000	2,049,000
Total, unrealized (losses)	$ (70,000)	$ (14,000)