Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of senior notes	$ 542,740	$ 542,740
Amount of difference between fair value and carrying value	(6,000)	(18,181)
Senior Notes Due 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of senior notes	292,740	292,740
Amount of difference between fair value and carrying value	0	(7,231)
Senior Notes Due 2021 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of senior notes	250,000	250,000
Amount of difference between fair value and carrying value	(6,000)	(10,950)
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	536,740	524,559
Fair Value, Measurements, Nonrecurring [Member] | Senior Notes Due 2020 [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	292,740	285,509
Fair Value, Measurements, Nonrecurring [Member] | Senior Notes Due 2021 [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	$ 244,000	$ 239,050