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Borrowings - Components of Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Interest Expense, Debt	$ 10,300	$ 10,500	$ 10,600
Outstanding Balance	200,000
Senior notes | Series B
Debt Instrument [Line Items]
Outstanding Balance			$ 50,000
Interest Rate Per Annum			5.32%
Senior notes | Series C
Debt Instrument [Line Items]
Outstanding Balance	$ 90,000
Interest Rate Per Annum	5.82%
Senior notes | Series D
Debt Instrument [Line Items]
Outstanding Balance	$ 60,000
Interest Rate Per Annum	4.29%
Senior notes | Series E Senior Notes
Debt Instrument [Line Items]
Outstanding Balance	$ 50,000
Interest Rate Per Annum	4.53%
Revolving credit agreement
Debt Instrument [Line Items]
Outstanding Balance	$ 0

X

- Definition

Amount of short-term and long-term debt and lease obligation.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense for 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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