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Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Related Party Transactions (Subcontracting Arrangements and Other Transactions) (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) employee	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) employee	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Management [Member] \| Subcontracting Arrangements [Member] \| Related Contractor or Supplier [Member]
Related Party Transaction [Line Items]
Number of additional management employees, subcontracting arrangement \| employee	2		2
Payments or expenses, related party	$ 1.8	$ 15.0	$ 6.8	$ 15.5
Payables, related party	2.3		2.3		$ 2.0
Management [Member] \| Service Agreements [Member]
Related Party Transaction [Line Items]
Payments or expenses, related party	4.4	7.9	13.3	14.5
Payables, related party	14.2		14.2		5.2
Management [Member] \| Lease Agreements [Member]
Related Party Transaction [Line Items]
Payments or expenses, related party	10.8	14.4	18.3	27.1
Payables, related party	3.0		3.0		0.1
Executive Officers [Member] \| Employee Leasing Arrangements [Member] \| Related Customer [Member]
Related Party Transaction [Line Items]
Charges, related party	0.2	0.2	0.4	0.4
Receivables, related party	0.2		0.2		0.2
Executive Officers [Member] \| Service Agreements [Member] \| Related Customer [Member]
Related Party Transaction [Line Items]
Revenue, related parties	0.2	0.2	0.5	0.4
Receivables, related party	0.3		0.3		$ 0.3
Chairman, Board of Directors [Member] \| Lease Agreements [Member] \| Aircraft [Member]
Related Party Transaction [Line Items]
Payments or expenses, related party	$ 0.5	$ 0.5	$ 1.1	$ 1.0