Noncontrolling Interest (Detail) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	9 Months Ended		9 Months Ended		9 Months Ended			0 Months Ended			0 Months Ended
	Jan. 25, 2013	Sep. 30, 2014	Jun. 25, 2014	Sep. 30, 2014 CyrusOne Inc. and CyrusOne GP [Member]	Jan. 23, 2013 CyrusOne Inc. and CyrusOne GP [Member]	Sep. 30, 2014 Cincinnati Bell Inc. [Member]	Sep. 30, 2014 Non- controlling Interest	Sep. 30, 2014 Additional Paid In Capital	Jun. 25, 2014 IPO [Member]	Jan. 25, 2013 IPO [Member]	Jun. 25, 2014 IPO [Member]	Jun. 25, 2014 Over-Allotment Option [Member]
Noncontrolling Interest [Line Items]
Number of outstanding partnership units purchased				21,900,000
Percentage of outstanding partnership units purchased				33.90%
Percentage of retained ownership						66.10%
Number of operating partnership units owned						42,600,000
Partnership units, conversion basis into common stock (in shares)		1
Common stock issued (in shares)	400,000								15,985,000	19,000,000		2,085,000
Ownership percentage						40.80%
Amount received from initial public offering of common stock, net of underwriter's discount									$ 371.7
Proceeds from issuance of initial public offering, net									355.9	337.1
Underwriting costs									15.8
Shares purchased of operating partnership's units			15,985,000		21,900,000
Redemption of noncontrolling interest							166.9	189.0
Redemption value of noncontrolling interests based on closing price (usd per share)	$ 19.00	$ 24.04									$ 23.25
Redemption value of noncontrolling interests		$ 639.5