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Investment in CyrusOne (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Jun. 30, 2014	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 24, 2013	Dec. 31, 2011
Assets Deconsolidated [Abstract]
Cash			$ 57.9	$ 4.6	$ 23.6		$ 73.7
Receivables			159.8us-gaap_AccountsReceivableNetCurrent	145.6us-gaap_AccountsReceivableNetCurrent
Other current assets			2.0us-gaap_OtherAssetsCurrent	1.6us-gaap_OtherAssetsCurrent
Property, plant and equipment			859.5us-gaap_PropertyPlantAndEquipmentNet	902.8us-gaap_PropertyPlantAndEquipmentNet
Other noncurrent assets			33.9us-gaap_OtherAssetsNoncurrent	36.6us-gaap_OtherAssetsNoncurrent
Total assets			1,819.7us-gaap_Assets	2,107.3us-gaap_Assets
Liabilities Deconsolidated [Abstract]
Current portion of long-term debt			13.2	12.6
Accounts payable			135.6us-gaap_AccountsPayableCurrent	89.4us-gaap_AccountsPayableCurrent
Unearned revenue and customer deposits			31.7us-gaap_DeferredRevenueCurrent	32.5us-gaap_DeferredRevenueCurrent
Other current liabilities			46.3us-gaap_OtherLiabilitiesCurrent	36.8us-gaap_OtherLiabilitiesCurrent
Long-term debt			1,771.0	2,252.6
Other noncurrent liabilities			47.3us-gaap_OtherLiabilitiesNoncurrent	74.4us-gaap_OtherLiabilitiesNoncurrent
Total liabilities			2,468.2us-gaap_Liabilities	2,784.0us-gaap_Liabilities
Equity Method Investment, Financial Statement, Reported Amounts [Abstract]
Net assets		509.7	0	509.7	0
Equity Method Investment Sold, Partnership Units	16.0
Sale of Partnership Units, Price Per Unit	$ 22.26cbb_SaleofPartnershipUnitsPricePerUnit
Proceeds from sale of CyrusOne equity method investment	355.9		355.9	0	0
Gain on sale of CyrusOne equity method investment	192.8		192.8	0	0
Total ownership interests in CyrusOne, Percentage			44.00%	68.00%		69.00%
Investment owned in CyrusOne Inc., Balance, Shares			1.9			1.9
Investment in CyrusOne LP, Balance, Partnership Units			26.6			42.6
Loss from CyrusOne			7.0	10.7	0
Equity Method Investments, Fair Value Disclosure			785.0	993.2
CyrusOne [Member]
Assets Deconsolidated [Abstract]
Cash						12.2
Receivables						41.5
Other current assets						13.4
Property, plant and equipment						736.2
Goodwill and intangibles						377.7
Other noncurrent assets						44.0
Total assets						1,225.0
Liabilities Deconsolidated [Abstract]
Current portion of long-term debt						6.3
Accounts payable						29.4
Unearned revenue and customer deposits						24.1
Other current liabilities						12.9
Long-term debt						550.3
Other noncurrent liabilities						92.3
Total liabilities						715.3
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Revenue			330.9	248.4
Operating income			40.0	28.9
Net loss			(14.5)	(15.6)
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Net investment in real estate			1,051.4	883.8
Total assets			1,586.5	1,506.8
Equity Method Investment, Summarized Financial Information, Liabilities [Abstract]
Total liabilities			$ 869.5	$ 729.2