Long-Term Debt - Net Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Disclosure [Abstract]
Interest expense	$ 8,041	$ 11,607	$ 16,340	$ 21,847
Interest income	(679)	(918)	(1,269)	(1,264)
Capitalized interest	(3,763)	(4,050)	(7,576)	(8,718)
Net interest expense	$ 3,599	$ 6,639	$ 7,495	$ 11,865