ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH

The following table summarizes the Company’s asset retirement obligations:



	December 31, 2017			December 31, 2016
Asset retirement obligation, beginning of period	$	17,033		$	8,573
Revision of estimate	249			4,186
Acquired in asset acquisitions or business combinations	—			5,454
Accretion of liabilities	1,733			906
Settlements	(735		)	(2,086		)
Asset retirement obligation, end of period	$	18,280		$	17,033
Asset retirement obligation:
Current	$	32		$	32
Non-current	18,248			17,001
Asset retirement obligation, end of period	$	18,280		$	17,033

The asset retirement obligations of the Company are subject to legal and regulatory requirements. Estimates of the costs of reclamation are reviewed periodically by the Company and the applicable regulatory authorities. The above provision represents the Company’s best estimate of the present value of future reclamation costs, discounted using credit adjusted risk-free interest rates ranging from 9.5% to 11.5% and an inflation rate of 2.0%. The total undiscounted decommissioning liability at December 31, 2017 is $43.46 million.

The following table summarizes the Company’s restricted cash:



	December 31, 2017			December 31, 2016
Restricted cash, beginning of period	$	23,175		$	12,980
Restricted cash from acquisitions	—			4,532
Refunds of collateral	(14,657		)	—
Additional collateral posted	13,609			5,663
Restricted cash, end of period	$	22,127		$	23,175

The Company has cash, cash equivalents and fixed income securities as collateral for various bonds posted in favor of the applicable state regulatory agencies in Arizona, Colorado, New Mexico, Texas, Utah and Wyoming, and the U.S. Bureau of Land Management and U.S. Forest Service for estimated reclamation costs associated with the White Mesa Mill, Nichols Ranch, Alta Mesa and other mining properties. Cash equivalents are short-term highly liquid investments with original maturities of three months or less. The restricted cash will be released when the Company has reclaimed a mineral property or restructured the surety and collateral arrangements. See Note 18 for a discussion of the Company’s surety bond commitments.