ASSET RETIREMENT OBLIGATION: (Tables)

3 Months Ended

Mar. 31, 2020

ASSET RETIREMENT OBLIGATION:

Summary of asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the three months ended March 31, 2020 and 2019 (in millions):

	2020		2019
Balance as of January 1	$	262.3	$	217.7
Changes in estimates		—		28.1
Closure payments		(0.6)		(0.2)
Accretion expense		3.8		3.0
Balance as of March 31,	$	265.5	$	248.6