Long-Term Debt - Net Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Debt Disclosure [Abstract]
Interest expense	$ 8,336	$ 10,745	$ 30,183	$ 34,224
Interest income	(792)	(833)	(2,056)	(1,713)
Capitalized interest	(3,929)	(3,069)	(12,647)	(7,504)
Net interest expense	$ 3,615	$ 6,843	$ 15,480	$ 25,007