Debt - Maturities for long-term debt and finance lease obligations and debt outstanding (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Maturities for long-term debt and capital lease obligations
2020	$ 217,440,000
2021	224,434,000
2022	230,312,000
2023	238,938,000
2024	644,538,000
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	9,726,428,000	$ 8,910,680,000
Long-term debt
Maturities for long-term debt and capital lease obligations
2020	211,359,000
2021	217,662,000
2022	222,771,000
2023	230,540,000
2024	$ 635,187,000
Weighted average interest rate on long-term debt	3.96%	4.24%
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	$ 9,726,428,000	$ 8,910,680,000
Unamortized Debt Issuance Costs and Debt Discounts	111,222,000	103,331,000
Finance Leases
Maturities for long-term debt and capital lease obligations
2020	6,081,000
2021	6,772,000
2022	7,541,000
2023	8,398,000
2024	9,351,000
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
2020	209,958,000
2021	216,652,000
2022	221,761,000
2023	229,530,000
2024	203,452,000
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	5,540,215,000	4,372,422,000
Unamortized Debt Issuance Costs and Debt Discounts	60,356,000	50,210,000
FMBs \| Long-term debt
Maturities for long-term debt and capital lease obligations
2020	1,010,000
2021	1,010,000
2022	1,010,000
2023	1,010,000
2024	63,510,000
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	3,205,052,000	3,556,062,000
Unamortized Debt Issuance Costs and Debt Discounts	39,793,000	41,509,000
PCBs \| Long-term debt
Maturities for long-term debt and capital lease obligations
2020	0
2021	0
2022	0
2023	0
2024	368,225,000
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	11,073,000	11,612,000
Series 2017 bonds \| Long-term debt
Maturities for long-term debt and capital lease obligations
2023	122,600,000
CFC \| Long-term debt
Maturities for long-term debt and capital lease obligations
2020	391,000
2021	0
2022	0
2023	0
2024	0
Long-term debt outstanding and the associated unamortized debt issuance costs and debt discounts
Principal	391,000	1,426,000
Unamortized Debt Issuance Costs and Debt Discounts	$ 0	$ 0

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 835
-SubTopic 30
-Section 55
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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
-Section 50
-Paragraph 4
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-Topic 835
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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 fourth 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 second 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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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