Asset Retirement Obligations

12 Months Ended

Dec. 31, 2017

Asset Retirement Obligation Disclosure [Abstract]

Asset Retirement Obligations

(10) Asset Retirement Obligations

The schedule below summarizes the changes in our asset retirement obligations (in millions):

	Year Ended December 31,
	2017		2016
Balance, beginning of period	$	13.5	$	14.0
Revisions to the fair values of existing liabilities	—		(0.5		)
Accretion expense	0.7		0.6
Liabilities settled	—		(0.6		)
Balance, end of period	$	14.2	$	13.5

Asset retirement obligations of $14.2 million and $13.5 million were included in “Asset retirement obligations” as non-current liabilities on the consolidated balance sheets as of December 31, 2017 and 2016, respectively.