Fair Value Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
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	(26,405)	(70,097)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	516,335	472,643
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	(13,905)	(26,347)
Senior Notes Due 2020 [Member] | 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	278,835	266,393
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	(12,500)	(43,750)
Senior Notes Due 2021 [Member] | 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	$ 237,500	$ 206,250