Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2017

Asset Retirement Obligation [Abstract]

Reconciliation of the company's asset retirement obligation

A reconciliation of the Company's liability, which is included in other accrued liabilities and deferred credits and other liabilities - other on the Consolidated Balance Sheets, for the years ended December 31 was as follows:

	2017			2016
	(In thousands)
Balance at beginning of year	$	314,970		$	242,224
Liabilities incurred	15,110			15,114
Liabilities settled	(4,981		)	(4,338		)
Accretion expense*	16,839			13,918
Revisions in estimates	31			48,052
Balance at end of year	$	341,969		$	314,970

* Includes $15.6 million and $12.7 million in 2017 and 2016, respectively, related to regulatory assets.