Derivative Instruments and Fair Value Measurements - Nonrecurring Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of investment in unconsolidated affiliate	$ 0	$ 2,500	$ 0	$ 2,500
VestaCare
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of investment in unconsolidated affiliate		2,500
Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of investment in unconsolidated affiliate				6,005
Fair Value, Measurements, Nonrecurring | AREC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment		3,200		3,200
Impairment of investment in unconsolidated affiliate				3,505
Fair Value, Measurements, Nonrecurring | AREC | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment		0		0
Fair Value, Measurements, Nonrecurring | AREC | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment		3,200		3,200
Fair Value, Measurements, Nonrecurring | AREC | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment		0		0
Fair Value, Measurements, Nonrecurring | VestaCare
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment		0		0
Impairment of investment in unconsolidated affiliate				2,500
Fair Value, Measurements, Nonrecurring | VestaCare | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment		0		0
Fair Value, Measurements, Nonrecurring | VestaCare | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment		0		0
Fair Value, Measurements, Nonrecurring | VestaCare | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment		$ 0		$ 0