Long-Term Debt, Carrying Value and Fair Value of Debt Outstanding (Details) - USD ($)		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		$ 1,475,040,000	$ 1,375,016,000
Fair value		1,570,693,000	1,421,325,000
Unamortized debt discounts and issuance costs		(6,454,000)	(5,249,000)	[1]
Long-term debt		1,468,586,000	1,369,767,000
Amount excluded on unamortized debt discounts and issuance costs			2,650,000
2.72% Senior Notes Due February 27, 2020 [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		0	150,000,000
Fair value		$ 0	151,547,000
Interest rate		2.72%
Maturity Date		Feb. 27, 2020
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		$ 364,538,000	353,216,000
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		581,115,000	541,546,000
Unamortized debt discounts and issuance costs		$ (705,000)
Interest rate		4.20%
Maturity Date		Mar. 01, 2028
Bank Note Payable [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		$ 40,000	16,000
Fair value		40,000	16,000
Revolving Credit Facility [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value	[2]	250,000,000	0
Fair value	[2]	$ 250,000,000	$ 0
Variable interest rates		1.50%	2.90%
Term Loan [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value	[2]	$ 375,000,000	$ 375,000,000
Fair value	[2]	375,000,000	$ 375,000,000
Long-term debt		$ 343,750,000
Variable interest rates		1.50%	2.90%
Credit Line [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value		$ 0	$ 0
Fair value		$ 0	$ 0

[1] Excludes $2,650,000 attributable to the Revolving Credit Facility included in other assets at December 31, 2019.

[2] Variable interest rate of 1.5% and 2.9% at December 31, 2020 and 2019, respectively.