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Principal Accounting Policies Self-Insurance Accruals (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Cash Equivalents and Marketable Securities [Abstract]      
Liability related to assumed claims $ 57.4us-gaap_AssumedLiabilityForUnpaidClaimsAndClaimsAdjustmentExpense $ 59.2us-gaap_AssumedLiabilityForUnpaidClaimsAndClaimsAdjustmentExpense  
Reinsurance receivables 43.3us-gaap_ReinsuranceReceivablesPaidLossesRecoverable 38.1us-gaap_ReinsuranceReceivablesPaidLossesRecoverable  
Operating income (loss) from reinsurance pool $ 6.1cnw_OperatingIncomeFromReinsurancePool $ 2.2cnw_OperatingIncomeFromReinsurancePool $ (2.5)cnw_OperatingIncomeFromReinsurancePool