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Stock Incentive Plans - Narrative (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Participant
Dec. 31, 2013
Dec. 31, 2012
May 20, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period                 3 years      
Total intrinsic value of stock options exercised in period                 $ 5.2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 2.3us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 2.2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue  
Total aggregate fair value of options vested in period                 0.3core_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedInPeriodTotalFairValue 0.2core_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedInPeriodTotalFairValue 0.1core_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedInPeriodTotalFairValue  
Stock options granted                 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross  
Stock-based compensation cost 1.8us-gaap_AllocatedShareBasedCompensationExpense 1.5us-gaap_AllocatedShareBasedCompensationExpense 1.5us-gaap_AllocatedShareBasedCompensationExpense 1.3us-gaap_AllocatedShareBasedCompensationExpense 0.6us-gaap_AllocatedShareBasedCompensationExpense 1.3us-gaap_AllocatedShareBasedCompensationExpense 1.4us-gaap_AllocatedShareBasedCompensationExpense 1.3us-gaap_AllocatedShareBasedCompensationExpense 6.1us-gaap_AllocatedShareBasedCompensationExpense 4.6us-gaap_AllocatedShareBasedCompensationExpense 5.8us-gaap_AllocatedShareBasedCompensationExpense  
Forfeiture rate since inception of plans                 4.00%core_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeitureRateSinceInceptionOfPlans      
Number of plan participants since inception of plans                 88core_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfPlanParticipantsSinceInceptionOfPlans      
Total unrecognized compensation cost related to non-vested share-based compensation arrangements 5.2us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized               5.2us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized      
Expected period to recognize total unrecognized compensation cost related to non-vested share-based compensation arrangements                 1 year 7 months 6 days      
Options [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award expiration period                 7 years      
Restricted stock units (RSUs) [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Total intrinsic value of equity instruments other than options exercised in period                 5.3core_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 7.6core_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 6.4core_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		  
Total aggregate fair value of equity instruments other than options vested in period                 5.4us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 8.5us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 6.4us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		  
Performance Shares [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Total intrinsic value of equity instruments other than options exercised in period                 1.1core_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 2.6core_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 1.1core_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		  
Total aggregate fair value of equity instruments other than options vested in period                 $ 1.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 $ 0.9us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 $ 1.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		  
2004 LTIP [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Shares cliff vest percentage                 33.30%core_ShareBasedCompensationArrangementByShareBasedPaymentAwardCliffVestPercentage
/ us-gaap_PlanNameAxis
= core_A2004LtipMember		      
2004 LTIP [Member] | Options [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Stock options granted                 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= core_A2004LtipMember		      
2005 LTIP [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Shares cliff vest percentage                 33.30%core_ShareBasedCompensationArrangementByShareBasedPaymentAwardCliffVestPercentage
/ us-gaap_PlanNameAxis
= core_A2005LtipMember		      
2007 LTIP [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Shares cliff vest percentage                 33.30%core_ShareBasedCompensationArrangementByShareBasedPaymentAwardCliffVestPercentage
/ us-gaap_PlanNameAxis
= core_A2007LtipMember		      
Remaining period following first vesting commencement date                 2 years      
2007 LTIP [Member] | Options [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Stock options granted                 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= core_A2007LtipMember		      
2010 LTIP [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares authorized                       3,131,904us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= core_A2010LtipMember
Annual award limits to any one participant                 200,000core_ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualAwardLimitsToAnyOneParticipant
/ us-gaap_PlanNameAxis
= core_A2010LtipMember		      
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Shares Available for Grant from Previous Plans 912,000core_SharebasedCompensationArrangementbySharebasedPaymentAwardMaximumSharesAvailableforGrantfromPreviousPlans
/ us-gaap_PlanNameAxis
= core_A2010LtipMember		               912,000core_SharebasedCompensationArrangementbySharebasedPaymentAwardMaximumSharesAvailableforGrantfromPreviousPlans
/ us-gaap_PlanNameAxis
= core_A2010LtipMember		      
2010 LTIP [Member] | Options [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Stock options granted                 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= core_A2010LtipMember		      
2010 LTIP [Member] | Restricted Stock Units and Performance Shares [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period                 3 years