Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Schedule of Changes in Asset Retirement Obligations

The following shows the changes in the asset retirement obligations:

years ended December 31:

(add 000)	2017			2016
Balance at beginning of year	$	101,106		$	89,604
Accretion expense		4,768			4,288
Liabilities incurred and liabilities assumed in business combinations		7,940			6,700
Liabilities settled		(309	)		166
Revisions in estimated cash flows		(3,852	)		348
Balance at end of year	$	109,653		$	101,106

Schedule of Contractual Purchase Commitments

The Company’s contractual purchase commitments as of December 31, 2017 are as follows:

(add 000)

2018	$	238,280
2019		37,275
2020		10,983
2021		8,809
2022		1,616
Thereafter		18,502
Total	$	315,465