Long-Term Debt, Carrying Value and Fair Value of Debt Outstanding (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value	$ 1,211,983,000	$ 1,475,040,000
Fair value	1,268,724,000	1,570,693,000
Unamortized debt discounts and issuance costs	(3,807,000)	(6,454,000)
Long-term Debt	1,208,176,000	1,468,586,000
Amount excluded on unamortized debt discounts and issuance costs	1,559,000
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	$ 357,848,000	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	$ 548,893,000	581,115,000
Interest rate	4.20%
Maturity Date	Mar. 01, 2028
Credit line due September 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	1,983,000	40,000
Fair value	1,983,000	40,000
Revolving Credit Facility [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value	0	250,000,000
Fair value	$ 0	$ 250,000,000
Variable interest rates	1.50%	1.50%
Maturity Date	Mar. 27, 2024
Term Loan [Member]
Carrying Value and Fair Value of Debt Outstanding [Abstract]
Carrying value	$ 360,000,000	$ 375,000,000
Fair value	360,000,000	$ 375,000,000
Long-term Debt	$ 20,000,000
Maturity Date	Mar. 27, 2024

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Amount, after accumulated amortization, of debt discount.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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