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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 |  |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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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 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6036836-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 + Details
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