Debt and Financing Arrangements Debt Disclosure (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	May 08, 2020
Debt Instrument [Line Items]
Total long-term debt, including current portion	$ 5,286,643,000	$ 5,286,643,000	$ 4,098,480,000
Other financing obligation	1,104,143,000	1,104,143,000	1,115,980,000
Unamortized debt issuance costs	(32,505,000)	(32,505,000)	(22,876,000)
Unamortized debt premium and discount, net	(14,358,000)	(14,358,000)	(10,634,000)
Long-term debt due within one year	22,640,000	22,640,000	20,180,000
Total long-term debt	5,217,140,000	5,217,140,000	4,044,790,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2021	22,640,000	22,640,000
2022	24,762,000	24,762,000
2023	27,082,000	27,082,000
2024	29,621,000	29,621,000
2025	32,397,000	32,397,000
3.25% due 2030 [Member]
Debt Instrument [Line Items]
Long-term debt face amount				$ 700,000,000
Long-term debt interest rate				3.25%
3.95 due 2050 [Member]
Debt Instrument [Line Items]
Long-term debt face amount				$ 500,000,000
Long-term debt interest rate				3.95%
Dalton Expansion Project [Member]
Debt Instrument [Line Items]
Other financing obligation			700,000
Long-term debt face amount	254,400,000	254,400,000	256,800,000
Other Long-term Debt, Current	2,300,000	2,300,000	2,100,000
Atlantic Sunrise Project [Member]
Debt Instrument [Line Items]
Other financing obligation	827,100,000	827,100,000	839,000,000.0
Additions to other financing obligations	25,000,000	8,700,000	40,800,000
Other Long-term Debt, Current	20,300,000	20,300,000	18,100,000
Property pledged as collateral
Debt Instrument [Line Items]
Total long-term debt	$ 0	$ 0
9.0 % Other Financing Obligations [Member]
Debt Instrument [Line Items]
Long-term debt interest rate	9.00%	9.00%
Debentures
Debt Instrument [Line Items]
Total long-term debt, including current portion	$ 207,500,000	$ 207,500,000	207,500,000
Debentures | 7.08% due 2026 [Member]
Debt Instrument [Line Items]
Total long-term debt, including current portion	7,500,000	7,500,000	7,500,000
Debentures | 7.25% due 2026 [Member]
Debt Instrument [Line Items]
Total long-term debt, including current portion	200,000,000	200,000,000	200,000,000
Notes
Debt Instrument [Line Items]
Total long-term debt, including current portion	3,975,000,000	3,975,000,000	2,775,000,000
Notes | 7.85% due 2026
Debt Instrument [Line Items]
Total long-term debt, including current portion	1,000,000,000	1,000,000,000	1,000,000,000
Notes | 4.0% due 2028 [Member]
Debt Instrument [Line Items]
Total long-term debt, including current portion	400,000,000	400,000,000	400,000,000
Notes | 3.25% due 2030 [Member]
Debt Instrument [Line Items]
Total long-term debt, including current portion	700,000,000	700,000,000	0
Notes | 5.4% due 2041 [Member]
Debt Instrument [Line Items]
Total long-term debt, including current portion	375,000,000	375,000,000	375,000,000
Notes | 4.45% due 2042 [Member]
Debt Instrument [Line Items]
Total long-term debt, including current portion	400,000,000	400,000,000	400,000,000
Notes | 4.6 % due 2048 [Member]
Debt Instrument [Line Items]
Total long-term debt, including current portion	600,000,000	600,000,000	600,000,000
Notes | 3.95 due 2050 [Member]
Debt Instrument [Line Items]
Total long-term debt, including current portion	$ 500,000,000	$ 500,000,000	$ 0