Long-Term Debt, Carrying Value and Fair Value of Debt Outstanding (Details) - USD ($)		3 Months Ended	12 Months Ended
		Mar. 31, 2021	Dec. 31, 2020
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		$ 1,355,434,000	$ 1,475,040,000
Fair value		1,412,735,000	1,570,693,000
Unamortized debt discounts and issuance costs		(6,096,000)	(6,454,000)
Long-term Debt		1,349,338,000	1,468,586,000
3.29% Senior Notes Due February, 27 2023 [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		350,000,000	350,000,000
Fair value		$ 360,344,000	364,538,000
Interest rate		3.29%
Maturity Date		Feb. 27, 2023
4.2% Senior Notes Due March 1, 2028 [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		$ 500,000,000	500,000,000
Fair value		$ 546,957,000	581,115,000
Interest rate		4.20%
Maturity Date		Mar. 01, 2028
Credit line due June 30, 2021 [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		$ 0	0
Fair value		0	0
Bank Note Payable [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		434,000	40,000
Fair value		434,000	40,000
Revolving Credit Facility [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value	[1]	130,000,000	250,000,000
Fair value	[1]	$ 130,000,000	$ 250,000,000
Variable interest rates		1.50%	1.50%
Term Loan [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value	[1]	$ 375,000,000	$ 375,000,000
Fair value	[1]	375,000,000	$ 375,000,000
Long-term Debt		$ 343,750,000
Variable interest rates		1.50%	1.50%

[1] Variable interest rate of 1.5% at both March 31, 2021 and December 31, 2020.