Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2015

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)	2015			2014
Balance at beginning of year	$	70,422		$	48,727
Accretion expense		3,336			2,818
Liabilities incurred and assumed in business combinations		14,735			20,984
Liabilities settled		(4,490	)		(2,061	)
Revisions in estimated cash flows		5,601			(46	)
Balance at end of year	$	89,604		$	70,422

Schedule of Contractual Purchase Commitments

The Corporation’s contractual purchase commitments as of December 31, 2015 are as follows:

(add 000)

2016	$	155,525
2017		6,457
2018		1,361
2019		451
2020		451
Thereafter		1,575
Total	$	165,820