Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2016

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)	2016		2015
Balance at beginning of year	$	89,604	$	70,422
Accretion expense		4,288		3,336
Liabilities incurred and liabilities assumed in business combinations		6,700		14,735
Liabilities settled		166		(4,490	)
Revisions in estimated cash flows		348		5,601
Balance at end of year	$	101,106	$	89,604

Schedule of Contractual Purchase Commitments

The Corporation’s contractual purchase commitments as of December 31, 2016 are as follows:

(add 000)

2017	$	154,804
2018		24,555
2019		16,453
2020		2,979
2021		454
Thereafter		1,136
Total	$	200,381