Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
General and administrative expense	$ (97,786)	$ (85,633)	$ (67,996)
Loss before income taxes	(228,435)	(659,599)	(187,273)
Income tax benefit (expense)	328,828	242,475	73,382
NET INCOME (LOSS)	100,393	(417,124)	(113,891)
Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Loss before income taxes	2,733	(111)	0
Income tax benefit (expense)	(1,010)	41	0
NET INCOME (LOSS)	1,723	(70)	0
Amortization of prior service cost | Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
General and administrative expense	$ 2,733	$ (111)	$ 0