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Fair Value Measurements (Assets Measured on a Nonrecurring Basis) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property and property held for sale gains (losses)			$ (2,000)		$ (7,800)
Property held for sale, impairment			(400)
Impairment loss	$ (2,398)	$ 0	(2,398)	$ (15,012)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant, and equipment, fair value disclosure	9,292		9,292		17,085
Property held for sale, fair value disclosure	39,193		39,193
Assets at fair value disclosure	48,485		48,485		17,085
Property and property held for sale gains (losses)			(1,950)		(7,828)
Property held for sale, impairment			(448)
Impairment loss			(2,398)		(7,828)
Fair Value, Measurements, Nonrecurring [Member] \| Quoted Prices In Active Markets For Identical Assets And Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value disclosure	0		0		0
Fair Value, Measurements, Nonrecurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant, and equipment, fair value disclosure	9,292		9,292		12,901
Property held for sale, fair value disclosure	39,193		39,193
Assets at fair value disclosure	48,485		48,485		12,901
Fair Value, Measurements, Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant, and equipment, fair value disclosure					4,184
Assets at fair value disclosure	0		0		4,184
Carrying Value [Member] \| Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant, and equipment, fair value disclosure	11,300		11,300		24,900
Property held for sale, fair value disclosure	39,600		39,600
Fair Value [Member] \| Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant, and equipment, fair value disclosure	9,300		9,300		$ 17,100
Property held for sale, fair value disclosure	$ 39,200		$ 39,200