PPAs and other definite-lived intangible assets and liabilities (Tables)

12 Months Ended

Dec. 31, 2020

PPAs and other definite-lived intangible assets and liabilities

Schedule of components of intangible assets subject to amortization

Assets

	Other Intangible Assets, Net
	Power Purchase
	Agreements		Total
Gross balances, January 1, 2020	$	365.6	$	365.6
Write-off of fully amortized balances		(13.5)		(13.5)
Gross balances, December 31, 2020		352.1		352.1
Less: accumulated amortization		(231.8)		(231.8)
Net carrying amounts, December 31, 2020	$	120.3	$	120.3

	Other Intangible Assets, Net
	Power Purchase
	Agreements		Total
Gross balances, December 31, 2019	$	365.6	$	365.6
Less: accumulated amortization		(221.3)		(221.3)
Net carrying amounts, December 31, 2019	$	144.3	$	144.3

Liabilities

	Power Purchase and Fuel Supply Agreement Liabilities, Net
	Power Purchase		Fuel Supply
	Agreements		Agreements		Total
Gross balances, December 31, 2020	$	(28.5)	$	(12.6)	$	(41.1)
Less: accumulated amortization		16.2		6.9		23.1
Net carrying amounts, December 31, 2020	$	(12.3)	$	(5.7)	$	(18.0)

	Power Purchase and Fuel Supply Agreement Liabilities, Net
	Power Purchase		Fuel Supply
	Agreements		Agreements		Total
Gross balances, December 31, 2019	$	(28.1)	$	(12.6)	$	(40.7)
Less: accumulated amortization		14.4		6.5		20.9
Net carrying amounts, December 31, 2019	$	(13.7)	$	(6.1)	$	(19.8)

Schedule of amortization expense of intangible assets

	2020		2019
PPAs	$	22.5	$	26.4
Fuel supply agreements		(0.4)		(0.4)
Total amortization	$	22.1	$	26.0

Schedule of estimated future amortization expense for the next five years

Year Ended December 31,
2021	$	20.2
2022		15.9
2023		12.6
2024		12.6
2025		12.6