Fair Value Disclosures (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements, Recurring and Nonrecurring

The following tables summarize the carrying amounts, fair values and fair value measurements recorded or disclosed as of December 31, 2016 and 2017, based on the three levels established by ASC 820; Fair Value Measurements and Disclosures (in thousands):

							Fair Value Measurements as of December 31, 2016 using:
Assets (Liabilities)	Carrying Amount			Fair Value			Quoted Prices in Active Markets for Identical Assets (Level 1)			Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (Level 3)
Energy commodity derivatives contracts	$	(30,738	)	$	(30,738	)	$	(30,738	)	$	—		$	—
Interest rate contracts	$	14,114		$	14,114		$	—		$	14,114		$	—
Long-term receivables	$	23,870		$	23,870		$	—		$	—		$	23,870
Debt	$	(4,087,192	)	$	(4,262,321	)	$	—		$	(4,262,321	)	$	—

							Fair Value Measurements as of December 31, 2017 using:
Assets (Liabilities)	Carrying Amount			Fair Value			Quoted Prices in Active Markets for Identical Assets (Level 1)			Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (Level 3)
Energy commodity derivatives contracts	$	(26,085	)	$	(26,085	)	$	(26,085	)	$	—		$	—
Interest rate contracts	$	12,177		$	12,177		$	—		$	12,177		$	—
Long-term receivables	$	27,676		$	27,676		$	—		$	—		$	27,676
Debt	$	(4,524,492	)	$	(4,826,480	)	$	—		$	(4,826,480	)	$	—