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Investments in debt and equity securities - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Investment
Dec. 31, 2013
Investment
Unrealized Losses On Investments [Line Items]    
Number of investments in an unrealized loss position 48us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 129us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Foreign Government Debt | Canada    
Unrealized Losses On Investments [Line Items]    
Foreign debt securities 135.8stc_DebtSecurities
/ invest_InvestmentAxis
= country_CA
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtMember		 133.8stc_DebtSecurities
/ invest_InvestmentAxis
= country_CA
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtMember
Foreign Government Debt | United Kingdom    
Unrealized Losses On Investments [Line Items]    
Foreign debt securities 27.3stc_DebtSecurities
/ invest_InvestmentAxis
= country_GB
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtMember		 25.2stc_DebtSecurities
/ invest_InvestmentAxis
= country_GB
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtMember