ASSET RETIREMENT OBLIGATION: (Tables)

9 Months Ended

Sep. 30, 2015

ASSET RETIREMENT OBLIGATION:

Summary of the asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the nine months ended September 30, 2015 and 2014 (in millions):

	2015			2014
Balance as of January 1	$	116.1		$	124.8
Changes in estimates	—			26.7
Closure payments	(15.3		)	(8.1		)
Accretion expense	13.1			5.8
Balance as of September 30,	$	113.9		$	149.2