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Debt - Lines of Credit and Other Debt Facilities (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Dec. 31, 2020
Debt Instrument |  |
Total debt and capital lease obligations | $ 1,514,400,000 | $ 1,515,500,000
Revolving Credit Facility |  |
Debt Instrument |  |
Reduction in borrowings | 5,500,000 | 5,500,000
Line of Credit |  |
Debt Instrument |  |
Outstanding letters of credit | 5,900,000 | 6,200,000
Line of Credit | Lines of Credit and Revolving Lines of Credit |  |
Debt Instrument |  |
Total debt and capital lease obligations | 0 | 0
Remaining borrowing capacity | $ 349,000,000 | $ 349,000,000

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- Definition Line of Credit Facility, Reduction to Borrowing Base From Outstanding Letters of Credit + References No definition available. + Details
Name: | esi_LineofCreditFacilityReductiontoBorrowingBaseFromOutstandingLettersofCredit
Namespace Prefix: | esi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + 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 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of long-term debt and lease obligation, including portion classified as current. + References No definition available. + Details
Name: | us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=esi_LinesofCreditandRevolvingLinesofCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |