Investment in CyrusOne - Schedule of Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CyrusOne [Member]
Revenue:
Services provided to CyrusOne		$ 1.3	$ 1.7
Operating costs and expenses:
Charges for services provided by CyrusOne		10.2	9.1
Administrative services provided to CyrusOne		(0.4)	(0.5)
Total operating costs and expenses		9.8	8.6
Balance Sheet Related Disclosures [Abstract]
Accounts receivable		0.1
Dividends receivable		2.1	$ 6.0
Receivable from CyrusOne		2.2
Payable to CyrusOne		$ 1.5
CyrusOne [Member]
Revenue:
Services provided to CyrusOne	$ 1.2
Operating costs and expenses:
Charges for services provided by CyrusOne	10.2
Administrative services provided to CyrusOne	(0.3)
Total operating costs and expenses	9.9
Balance Sheet Related Disclosures [Abstract]
Accounts receivable	0.0
Dividends receivable	1.1
Receivable from CyrusOne	1.1
Payable to CyrusOne	$ 0.9