ASSET RETIREMENT OBLIGATION: (Tables)

3 Months Ended

Mar. 31, 2014

ASSET RETIREMENT OBLIGATION:

Summary of the asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the three months ended March 31, 2014 and 2013 (in millions):

	2014			2013
Balance as of January 1	$	124.8		$	122.3
Changes in estimates	26.7			—
Additions	—			—
Payments	(1.5		)	(1.9		)
Accretion expense	2.0			1.8
Balance as of March 31,	$	152.0		$	122.2