Investment in CyrusOne - Schedule of Related Party Transactions (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating costs and expenses:
Transaction-related compensation to CyrusOne employees		$ 20.0
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital	28.4	21.3	0
Balance Sheet Related Disclosures [Abstract]
Dividends receivable	6.0	7.1	0
Receivable from CyrusOne	7.7	9.2
Payable to CyrusOne	0.4	0.5
CyrusOne [Member]
Revenue:
Services provided to CyrusOne	1.7	2.1
Operating costs and expenses:
Transaction-related compensation to CyrusOne employees	0	20.0
Charges for services provided by CyrusOne	9.1	8.8
Administrative services provided to CyrusOne	(0.5)	(0.6)
Total operating costs and expenses	8.6	28.2
Income Tax Receivable from Related Party	1.7	1.5
Balance Sheet Related Disclosures [Abstract]
Accounts receivable	1.7	2.1
Dividends receivable		7.1
Receivable from CyrusOne	7.7	9.2
Accounts payable	0.4	0.5
Payable to CyrusOne	$ 0.4	$ 0.5
CyrusOne [Member]
Operating costs and expenses:
Dividends Payable, Amount Per Share	$ 0.21