Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Transaction price	$ 615,000
Revenue, remaining performance obligation, expected timing of satisfaction, period	24 years
Contract revenue	$ 117,814
Contract with Customer, Asset and Liability [Abstract]
Accounts receivable	122,090	$ 143,653
Deferred costs	3,300	2,900
Contract with Customer, Liability [Abstract]
Deferral of revenue	5,761
Recognition of revenue included in beginning of year contract liability	(12,302)
Contract liability	66,329
Accounting Standards Update 2014-09 [Member]
Contract with Customer, Liability [Abstract]
Contract liability	72,870
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Contract with Customer, Liability [Abstract]
Contract liability	42,379
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Contract with Customer, Liability [Abstract]
Contract liability	30,491
ADM [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings		32,500
Deferred Tax Assets		$ 9,900
Contract with Customer, Liability [Abstract]
Contract liability	$ 42,400