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Fair-Value Measurements (Reconciliation of Changes in Derivatives Classified as Level 3) (Details) - Level 3 - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Beginning Balance $ (45,209) $ 0
Initial valuation of Preferred Distribution Rate Reset Election   (34,450)
Net (loss) gain for the period included in earnings 8,360 (10,472)
Allocation of Distribution Paid-in-kind (3,991) (287)
Ending Balance $ (40,840) $ (45,209)