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Changes in shareholders' equity - additional information (Details) (USD $)
Share data in Millions, except Per Share data, unless otherwise specified
3 Months Ended
Mar. 29, 2015
Mar. 30, 2014
Jun. 30, 2007
Dec. 31, 2014
Shareholders Equity [Line Items]        
Repurchase of outstanding common stock, authorized amount     $ 300,000,000us-gaap_StockRepurchaseProgramAuthorizedAmount1  
Debt instrument, stated interest rate 3.875%us-gaap_DebtInstrumentInterestRateStatedPercentage 3.875%us-gaap_DebtInstrumentInterestRateStatedPercentage    
Equity Option        
Shareholders Equity [Line Items]        
Weighted average antidilutive which were not included in the calculation of earnings per share 5.8us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
6.5us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
   
Convertible Note        
Shareholders Equity [Line Items]        
Weighted average antidilutive which were not included in the calculation of earnings per share 3.1us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember
2.5us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember
   
Per share market price of common stock       $ 61.32us-gaap_DebtInstrumentConvertibleConversionPrice1
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember
Strike price of warrents 74.65invest_InvestmentWarrantsExercisePrice
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember
     
Conversion of convertible notes, shares issued upon conversion 2.3us-gaap_DebtConversionConvertedInstrumentSharesIssued1
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember
1.6us-gaap_DebtConversionConvertedInstrumentSharesIssued1
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember