Long-Term Debt - Net Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Disclosure [Abstract]
Interest expense	$ 8,171	$ 8,336	$ 24,511	$ 30,183
Interest income	(994)	(792)	(2,263)	(2,056)
Capitalized interest	(3,928)	(3,929)	(11,504)	(12,647)
Net interest expense	$ 3,249	$ 3,615	$ 10,744	$ 15,480