Senior Notes (Schedule Of Outstanding Senior Notes) (Details) - Senior Notes [Member] - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Senior Notes 4.868% [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Date Issued	Jun. 05, 2014
Date Due	Jun. 01, 2044	Jun. 01, 2044
Principal Amounts	$ 400,000,000
Consolidated Balance Sheet Amount	397,254,000	$ 397,194,000
Market Value	$ 505,284,000	$ 528,000,000
Debt instrument, stated interest rate	4.868%
Senior Notes 3.5% [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Date Issued	Oct. 07, 2020
Date Due	Oct. 15, 2050	Oct. 15, 2050
Principal Amounts	$ 1,000,000,000
Consolidated Balance Sheet Amount	979,784,000	$ 979,524,000
Market Value	$ 1,068,990,000	$ 1,138,100,000
Debt instrument, stated interest rate	3.50%