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Employee Benefit Plans (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]      
Overall expected long-term rate of return 5.15%cnw_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongtermReturnonAssetsintheNextFiscalYear    
Defined Contribution Plan, Cost Recognized $ 56,300,000us-gaap_DefinedContributionPlanCostRecognized $ 55,300,000us-gaap_DefinedContributionPlanCostRecognized $ 50,800,000us-gaap_DefinedContributionPlanCostRecognized
Fixed-Income Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Investment strategy mix, securities 76.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
   
Equity Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Investment strategy mix, securities 24.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
   
U S Large Companies [Member] | Equity Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Investment strategy mix, securities 12.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ invest_InvestmentGeographicRegionAxis
= cnw_USLargeCompaniesMember
   
U S Small Companies [Member] | Equity Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Investment strategy mix, securities 2.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ invest_InvestmentGeographicRegionAxis
= cnw_USSmallCompaniesMember
   
International | Equity Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Investment strategy mix, securities 10.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ invest_InvestmentGeographicRegionAxis
= cnw_InternationalMember
   
Postretirement Medical Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan amendments 0us-gaap_DefinedBenefitPlanPlanAmendments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
(19,243,000)us-gaap_DefinedBenefitPlanPlanAmendments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
 
Qualified Pension Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan settlement 36,237,000us-gaap_DefinedBenefitPlanSettlementsBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cnw_QualifiedPensionPlansMember
0us-gaap_DefinedBenefitPlanSettlementsBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cnw_QualifiedPensionPlansMember
 
Settlement loss 15,965,000us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cnw_QualifiedPensionPlansMember
0us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cnw_QualifiedPensionPlansMember
0us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cnw_QualifiedPensionPlansMember
Estimated future employer contributions in next fiscal year 30,000,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cnw_QualifiedPensionPlansMember
   
Impact on Projected Benefit Obligation [Member] | Postretirement Medical Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan amendments   28,300,000us-gaap_DefinedBenefitPlanPlanAmendments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ cnw_ImpactofPlanAmendmentAxis
= cnw_ImpactonProjectedBenefitObligationMember
 
Impact on Actuarial Gain [Member] | Postretirement Medical Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan amendments   $ 9,100,000us-gaap_DefinedBenefitPlanPlanAmendments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ cnw_ImpactofPlanAmendmentAxis
= cnw_ImpactonActuarialGainMember
 
Global opportunistic [Member] | Fixed-Income Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Investment strategy mix, securities 7.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= cnw_GlobalopportunisticMember