ASSET RETIREMENT OBLIGATION: (Tables)

9 Months Ended

Sep. 30, 2014

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, 2014 and 2013 (in millions):

	2014			2013
Balance as of January 1	$	124.8		$	122.3
Changes in estimates	26.7			—
Closure payments	(8.1		)	(2.3		)
Accretion expense	5.8			6.7
Balance as of September 30,	$	149.2		$	126.7