Debt - Maturities for long-term debt and finance lease obligations and debt outstanding (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Maturities for long-term debt and capital lease obligations
2021	$ 208,649,000
2022	284,104,000
2023	379,629,000
2024	540,739,000
2025	302,081,000
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	10,619,826,000	$ 9,726,428,000
Long-term debt
Maturities for long-term debt and capital lease obligations
2021	201,877,000
2022	276,563,000
2023	371,231,000
2024	531,388,000
2025	$ 291,668,000
Weighted average interest rate on long-term debt	3.73%	3.96%
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	$ 10,619,826,000	$ 9,726,428,000
Unamortized Debt Issuance Costs and Debt Discounts	119,565,000	111,222,000
Finance Leases
Maturities for long-term debt and capital lease obligations
2021	6,772,000
2022	7,541,000
2023	8,398,000
2024	9,351,000
2025	10,413,000
Credit Facility
Maturities for long-term debt and capital lease obligations
Principal amount	1,210,000,000
FFB \| Long-term debt
Maturities for long-term debt and capital lease obligations
2021	200,867,000
2022	275,553,000
2023	247,601,000
2024	222,273,000
2025	229,168,000
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	5,985,015,000	5,540,215,000
Unamortized Debt Issuance Costs and Debt Discounts	57,971,000	60,356,000
FMBs \| Long-term debt
Maturities for long-term debt and capital lease obligations
2021	1,010,000
2022	1,010,000
2023	1,010,000
2024	63,510,000
2025	62,500,000
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	3,654,041,000	3,205,052,000
Unamortized Debt Issuance Costs and Debt Discounts	49,134,000	39,793,000
PCBs \| Long-term debt
Maturities for long-term debt and capital lease obligations
2021	0
2022	0
2023	122,620,000
2024	245,605,000
2025	0
Principal amount	245,600,000
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	980,770,000	980,770,000
Unamortized Debt Issuance Costs and Debt Discounts	12,460,000	11,073,000
Series 2017 bonds \| Long-term debt
Maturities for long-term debt and capital lease obligations
2023	122,600,000
CFC \| Long-term debt
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	0	391,000
Unamortized Debt Issuance Costs and Debt Discounts	$ 0	$ 0

Face (par) amount of debt instrument at time of issuance.

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-Name Accounting Standards Codification
-Topic 835
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount of unamortized debt discount (premium) and debt issuance costs.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
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Principal amount of long-term debt and capital lease obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Principal amount of long-term debt and capital lease obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Principal amount of long-term debt and capital lease obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Weighted average interest rate of long-term debt outstanding calculated at point in time.

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Carrying amount of collateralized debt obligations with maturities initially due after one year or beyond the operating cycle, if longer, excluding the current portion. Obligations include, but not limited to, mortgage loans, chattel loans, and other borrowings secured by assets.

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