Intangible Assets, Goodwill and Other Assets (Schedule of Other Assets) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
CO2 volumetric production payments, net of amortization		$ 2,175	$ 3,503
Deferred marine charges, net	[1]	30,246	27,710
Other deferred costs and deposits		24,207	25,398
Other assets, net of amortization		$ 56,628	$ 56,611

[1] (1)See discussion of deferred charges on marine transportation assets in the Summary of Accounting Policies (Note 2)