Statements of Cash Flows (Cash Flow Supplemental Disclosures) (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Federal income tax		$ 34,000	$ 42,000
Operating cash flows from operating leases		7,077	6,062
Operating cash flows from financing leases		13	12
Financing cash flows from finance leases		429	811
Assets acquired under finance lease arrangements		119	824
Assets acquired under operating lease arrangements		22,104	13,648
Non-cash purchase of property and equipment		9	108
Interest Paid, Operating and Investing Activities		24,449	19,261
Available-for-sale Securities
Corporate actions related to securities	[1]	32,580	29,771
Equity Securities [Member]
Corporate actions related to securities	[1]	890	14,250
Held-to-maturity Securities [Member]
Corporate actions related to securities	[1]	$ 2,596	$ 0

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