Statements of Cash Flows (Cash Flow Supplemental Disclosures) (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020
Interest		$ 23,278	$ 24,449
Federal income tax		93,000	34,000
Operating cash flows from operating leases		6,480	7,077
Operating cash flows from financing leases		5	13
Financing cash flows from finance leases		298	429
Assets acquired under finance lease arrangements		183	119
Assets acquired under operating lease arrangements		273	22,104
Non-cash purchase of property and equipment		0	9
Available-for-sale Securities
Corporate actions related to securities	[1]	50,501	32,580
Equity Securities [Member]
Corporate actions related to securities	[1]	527	890
Held-to-maturity Securities [Member]
Corporate actions related to securities	[1]	$ 0	$ 2,596

[1] Examples of corporate actions include exchanges, non-cash acquisitions, and stock splits.