ASSET RETIREMENT OBLIGATION: (Tables)

12 Months Ended

Dec. 31, 2013

ASSET RETIREMENT OBLIGATION:

Summary of the asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the two years ended December 31, 2013 and 2012 (in millions):

	2013			2012
Balance as of January 1	$	118.2		$	62.0
Changes in estimates	—			27.4
Additions	—			25.1
Closure payments	(1.5		)	(0.3		)
Accretion expense	8.1			4.0
Balance as of December 31,	$	124.8		$	118.2