CONDENSED CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Revenues:
Product sales	$ 772,931us-gaap_SalesRevenueGoodsNet	$ 662,741us-gaap_SalesRevenueGoodsNet
Product sales to related party	21,256us-gaap_RevenueFromRelatedParties	20,700us-gaap_RevenueFromRelatedParties
Commissions	4,353us-gaap_ContractsRevenue	3,818us-gaap_ContractsRevenue
Total revenues	798,540us-gaap_Revenues	687,259us-gaap_Revenues
Cost of product sales	698,832us-gaap_CostOfGoodsSold	598,171us-gaap_CostOfGoodsSold
Gross profit	99,708us-gaap_GrossProfit	89,088us-gaap_GrossProfit
Selling, general and administrative expenses	63,888	56,073
Depreciation and amortization	3,108us-gaap_DepreciationAndAmortization	2,809us-gaap_DepreciationAndAmortization
Operating income	32,712us-gaap_OperatingIncomeLoss	30,206us-gaap_OperatingIncomeLoss
Other income (expense):
Interest expense	(253)us-gaap_InterestExpense	(239)us-gaap_InterestExpense
Earnings of equity method investees	108	104
Other	33us-gaap_OtherNonoperatingIncomeExpense	197us-gaap_OtherNonoperatingIncomeExpense
Total other income (expense), net	(112)us-gaap_NonoperatingIncomeExpense	62us-gaap_NonoperatingIncomeExpense
Income before taxes	32,600mwiv_IncomeBeforeTaxes	30,268mwiv_IncomeBeforeTaxes
Income tax expense	(12,545)us-gaap_IncomeTaxExpenseBenefit	(11,829)us-gaap_IncomeTaxExpenseBenefit
Net income	$ 20,055us-gaap_ProfitLoss	$ 18,439us-gaap_ProfitLoss
Earnings Per Share, Basic and Diluted [Abstract]
Basic	$ 1.57us-gaap_EarningsPerShareBasic	$ 1.45us-gaap_EarningsPerShareBasic
Diluted	$ 1.57us-gaap_EarningsPerShareDiluted	$ 1.45us-gaap_EarningsPerShareDiluted
Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract]
Weighted Average Number of Shares Outstanding, Basic	12,750	12,707
Weighted Average Number of Shares Outstanding, Diluted	12,774	12,743

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