Long term debt (Details) - USD ($) $ in Millions				9 Months Ended	12 Months Ended
	Sep. 30, 2021	Aug. 11, 2021	Jan. 06, 2021	Sep. 30, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Mar. 11, 2020
Debt Instrument [Line Items]
Proceeds from Issuance of Long-term Debt				$ 400	$ 400	$ 549	$ 470
Long-term Debt, Current Maturities					0	160
Repayments of Long-term Debt		$ (20)	$ (140)		$ (160)	$ (98)	$ (350)
issuance of 2028 Bonds	$ 100
Issuance of 2034 Bonds	100
Issuance of 2051 Bonds	150
Pollution Control Revenue Bonds at 2,125%								$ 98
Pollution Control Revenue Bonds at 2.375%								$ 21
Issuance of 2031 Bonds	$ 50