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Goodwill and Other Assets - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Goodwill and Intangible Assets Disclosure [Abstract]      
Goodwill $ 2,776,188,000us-gaap_Goodwill $ 2,393,731,000us-gaap_Goodwill  
Change in goodwill carrying amount 382,500,000us-gaap_GoodwillPeriodIncreaseDecrease 228,100,000us-gaap_GoodwillPeriodIncreaseDecrease  
Goodwill deductible for tax purpose 229,300,000us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount    
Other intangible assets gross carrying amounts 299,000,000us-gaap_FiniteLivedIntangibleAssetsGross 174,400,000us-gaap_FiniteLivedIntangibleAssetsGross  
Accumulated amortization 100,900,000us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization 70,800,000us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization  
Amortization expense for existing other intangible assets 30,100,000us-gaap_AmortizationOfIntangibleAssets 24,500,000us-gaap_AmortizationOfIntangibleAssets 15,000,000us-gaap_AmortizationOfIntangibleAssets
Remaining weighted average amortization period of other intangible assets, in years 8 years 8 months 12 days    
Weighted average amortization period includes amortization expense related to years beyond 2019 69,100,000us-gaap_FiniteLivedIntangibleAssetsNet    
Other assets 37,289,000us-gaap_OtherAssetsMiscellaneousNoncurrent 25,799,000us-gaap_OtherAssetsMiscellaneousNoncurrent  
Carrying amount of cash surrender value of life insurance 16,000,000us-gaap_CashSurrenderValueOfLifeInsurance 17,100,000us-gaap_CashSurrenderValueOfLifeInsurance  
Investments in joint venture $ 9,500,000md_JointVentureInvestment