Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral posted	$ 0
6.625% senior subordinated notes due 2021	645,000	$ 644,500
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of long-term debt	626,000	600,500
Unit Credit Agreement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Line of credit facility, amount outstanding	40,000	0
Superior Credit Agreement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Line of credit facility, amount outstanding	$ 0	$ 0