ASSET RETIREMENT OBLIGATION: (Tables)

9 Months Ended

Sep. 30, 2020

ASSET RETIREMENT OBLIGATION:

Summary of asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the nine months ended September 30, 2020 and 2019 (in millions):

	2020		2019
Balance as of January 1	$	262.3	$	217.7
Changes in estimates		—		25.2
Closure payments		(1.0)		(0.8)
Accretion expense		11.1		9.5
Balance as of September 30,	$	272.4	$	251.6