Segment Information (Entity-Wide Disclosures) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Interest Expense	$ 5,366	$ 3,759	$ (10,728)	$ (7,307)
Customer revenues	203,607	253,211	406,647	491,128
Customer Revenues [Member]
Segment Reporting Information [Line Items]
Customer revenues	203,607	253,211	406,647	491,128
CAM [Member]
Segment Reporting Information [Line Items]
Interest Expense	4,500	3,500	9,000	6,900
Customer revenues	54,377	49,530	106,753	97,508
CAM [Member] | Customer Revenues [Member]
Segment Reporting Information [Line Items]
Customer revenues	38,016	32,380	73,903	63,162
All other [Member]
Segment Reporting Information [Line Items]
Customer revenues	19,730	50,172	39,013	99,040
All other [Member] | Customer Revenues [Member]
Segment Reporting Information [Line Items]
Customer revenues	16,933	45,173	33,772	81,942
Ground Services [Member] | All other [Member] | Customer Revenues [Member]
Segment Reporting Information [Line Items]
Customer revenues	$ 16,159	$ 44,650	$ 32,314	$ 80,949