Long-term debt - Narrative (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Cash and cash equivalents	$ 57,262,000	$ 40,968,000
Amended and Restated Competitive Advance and Revolving Credit Agreement \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Unused borrowing capacity	1,230,000,000
Maximum borrowing capacity under credit facility	$ 1,510,000,000

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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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.

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- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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