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Investments - Summary of Company's Investment Securities (Detail) (USD $)
In Millions, unless otherwise specified
6 Months Ended 12 Months Ended
Nov. 30, 2014
May 31, 2014
Summary of Investment Holdings [Line Items]    
Available-for-sale, Amortized Cost $ 10.7us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 10.4us-gaap_AvailableForSaleSecuritiesAmortizedCost
Available-for-sale, Unrealized Gains 0.1bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 0.6bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Available-for-sale, Unrealized Losses (0.1)bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax (0.3)bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Available-for-sale, Fair Value 10.7us-gaap_AvailableForSaleSecurities 10.7us-gaap_AvailableForSaleSecurities
Trading, Amortized Cost 2.7bmet_TradingSecuritiesAmortizedCost 1.6bmet_TradingSecuritiesAmortizedCost
Trading, Realized Gains 0.1us-gaap_TradingSecuritiesRealizedGain 0.3us-gaap_TradingSecuritiesRealizedGain
Trading, Realized Losses 0us-gaap_TradingSecuritiesRealizedLoss (0.1)us-gaap_TradingSecuritiesRealizedLoss
Trading, Fair Value 2.8us-gaap_TradingSecurities 1.8us-gaap_TradingSecurities
Equity securities [Member]    
Summary of Investment Holdings [Line Items]    
Available-for-sale, Amortized Cost 0.5us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
0.2us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Available-for-sale, Unrealized Gains 0.1bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
0.6bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Available-for-sale, Unrealized Losses 0bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(0.3)bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Available-for-sale, Fair Value 0.6us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
0.5us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Trading, Amortized Cost 2.7bmet_TradingSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1.6bmet_TradingSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Trading, Realized Gains 0.1us-gaap_TradingSecuritiesRealizedGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
0.3us-gaap_TradingSecuritiesRealizedGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Trading, Realized Losses 0us-gaap_TradingSecuritiesRealizedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(0.1)us-gaap_TradingSecuritiesRealizedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Trading, Fair Value 2.8us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1.8us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Time deposit [Member]    
Summary of Investment Holdings [Line Items]    
Available-for-sale, Amortized Cost 10.2us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bmet_TimeDepositsMember
10.2us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bmet_TimeDepositsMember
Available-for-sale, Unrealized Gains 0bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bmet_TimeDepositsMember
0bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bmet_TimeDepositsMember
Available-for-sale, Unrealized Losses (0.1)bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bmet_TimeDepositsMember
0bmet_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bmet_TimeDepositsMember
Available-for-sale, Fair Value $ 10.1us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bmet_TimeDepositsMember
$ 10.2us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bmet_TimeDepositsMember