Note 9 - Cash, Cash Equivalents and Investments (Details) - Current Assets Fair Market Value (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Cash, cash equivalents and investments:
Cash	$ 49,001us-gaap_Cash	$ 66,188us-gaap_Cash
Money market funds	46,965us-gaap_MoneyMarketFundsAtCarryingValue	60,078us-gaap_MoneyMarketFundsAtCarryingValue
Certificates of deposit	23,050	22,778
U.S. treasuries and government agency bonds	114,741	89,674
Total	239,151mpwr_CashCashEquivalentsAndInvestments	244,107mpwr_CashCashEquivalentsAndInvestments
Auction Rate Securities [Member]
Cash, cash equivalents and investments:
Auction-rate securities backed by student-loan notes	$ 5,394	$ 5,389

Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable. Long-term investments include auction-rate securities.

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A savings certificate entitling the Entity (that is, bearer) to receive interest at an established maturity date, based upon a fixed interest rate. A certificate of deposit may be issued in any denomination. Certificates of deposit are generally issued by commercial banks and, therefore, insured by the FDIC (up to the prescribed limit). Certificates of deposit generally restrict holders from withdrawing funds on demand without the incurrence of penalties. Generally, only certificates of deposit with original maturities of three months or less qualify as cash equivalents. Original maturity means original maturity to the entity holding the investment. As a related example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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