Statements of Cash Flow (Cash Flow Supplemental Disclosures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest	$ 30,464	$ 25,089	$ 23,992
Federal income tax	47,000	55,825	29,193
Operating cash flows from operating leases	9,498	8,138	0
Operating cash flows from financing leases	15	16	0
Financing cash flows from financing leases	550	977	5,646
Assets acquired under finance lease arrangements	324	824	4,119
Assets acquired under operating lease arrangements	22,390	13,808	0
Non-cash purchase of property and equipment	590	89	291
Available-for-sale Securities [Member]
Corporate actions related to securities	55,446	61,369	52,277
Held-to-maturity Securities [Member]
Corporate actions related to securities	2,589	0	0
Equity Securities [Member]
Corporate actions related to securities	$ 10,890	$ 14,250	$ 944