Interest Expense (Interest Expense Schedule) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Interest Expense [Abstract]
Contractual interest on Convertible Debentures	$ 14,948	$ 10,156
Contractual interest on Notes	21,766	15,235
Amortization of debt discount on the Convertible Debentures	3,161	2,911
Credit facility and other interest expense	913	721
Total interest expense	$ 40,788	$ 29,023