Actual tangible net worth.

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Debt instrument average maturity period.

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Maturity period of term loan facility under subtranche.

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Date when the first installment of the debt instrument is scheduled to be repaid, in CCYY-MM-DD format.

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Line of credit facility additional capacity available.

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Line of credit facility covenants actual aggregate fixed charge coverage ratio.

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Line of credit facility covenants aggregate fixed charge coverage ratio.

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Line of credit facility covenants aggregate fixed charge leverage ratio.

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Line of credit facility covenants tangible net worth component aggregate net income percent.

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Line of credit facility covenants tangible net worth component fixed amount.

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Line of credit facility covenants tangible net worth component proceeds from issuance of common stock percentage.

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Line of credit facility covenants tangible net worth description.

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Line of credit facility enhanced borrowing capacity.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Identification of the lender and information about a contractual promise to repay a short-term or long-term obligation, which includes borrowings under lines of credit, notes payable, commercial paper, bonds payable, debentures, and other contractual obligations for payment. This may include rationale for entering into the arrangement, significant terms of the arrangement, which may include amount, repayment terms, priority, collateral required, debt covenants, borrowing capacity, call features, participation rights, conversion provisions, sinking-fund requirements, voting rights, basis for conversion if convertible and remarketing provisions. The description may be provided for individual debt instruments, rational groupings of debt instruments, or by debt in total.

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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Face (par) amount of debt instrument at time of issuance.

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Description of the frequency of periodic payments (monthly, quarterly, annual).

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

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Amount of commitment fees for the unused borrowing capacity under the long-term financing arrangement that is available to the entity.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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Notes Payable - Additional Information (Detail) (USD $)	12 Months Ended	0 Months Ended	3 Months Ended
Notes Payable - Additional Information (Detail) (USD $)	Dec. 31, 2014	Mar. 17, 2014	Dec. 31, 2013	Apr. 21, 2011	Jun. 30, 2012
Debt Instrument [Line Items]
Consolidated Fixed Charge Coverage Ratio	2.50
Consolidated Fixed Charge Coverage Ratio, actual	4.02
Consolidated Leverage Ratio	2.75
Consolidated Leverage Ratio, actual	1.89
Line of credit facility covenants tangible net worth description	Permit Tangible Net Worth, calculated as of the last day of each fiscal quarter, to be less than the sum of (i) $229.0 million, plus (ii) 25% of net income for such fiscal quarter subsequent to December 31, 2010, plus (iii) 90% of the net cash proceeds from the issuance of the Company's capital stock after December 31, 2010.
Tangible Net Worth sum	$ 294,200,000
Consolidated Net Income	25.00%
Percentage of net cash proceeds from issuance of capital stock	90.00%
Actual tangible net worth	386,400,000mgrc_ActualTangibleNetWorth
Minimum [Member]
Debt Instrument [Line Items]
Tangible Net Worth sum	229,000,000
Senior Notes [Member] \| 4.03% senior notes due in 2018 [Member]
Debt Instrument [Line Items]
Aggregate principal amount				100,000,000
Senior notes interest rate	4.03%			4.03%
Debt instrument, payment terms	Interest on these notes is due semi-annually in arrears and the principal is due in five equal annual installments, with the first payment due on April 21, 2014.
Debt instrument, maturity date	Apr. 21, 2018
Debt instrument, date of first required payment	Apr. 21, 2014
Senior notes principal balance outstanding	80,000,000		100,000,000
Senior Notes [Member] \| 4.03% senior notes due in 2018 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, maturity period	12 years
Debt instrument, average life	10 years
Senior Notes [Member] \| 3.68% senior notes due in 2021 [Member]
Debt Instrument [Line Items]
Senior notes interest rate	3.68%	3.68%
Debt instrument, maturity date		Mar. 17, 2021
Proceeds from issuance of senior notes		40,000,000
Senior notes description		The Series B Senior Notes are an unsecured obligation of the Company and bear interest at a rate of 3.68% per annum and mature on March 17, 2021. Interest on the Series B Senior Notes is payable semi-annually beginning on September 17, 2014 and continuing thereafter on March 17 and September 17 of each year until maturity.
Senior notes interest payment		Semi-annually
Unsecured revolving credit facility before Amendment [Member]
Debt Instrument [Line Items]
Credit facility, maximum	350,000,000
Amended Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility, maturity period	5 years
Revolving credit facility, maturity date	Jun. 15, 2017
Credit facility, maximum	420,000,000
Revolving credit facility, available maximum borrowing capacity	450,000,000
Additional commitments	30,000,000
Amount outstanding	200,000,000		186,000,000
Consolidated Fixed Charge Coverage Ratio	2.50
Consolidated Fixed Charge Coverage Ratio, actual	4.02
Consolidated Leverage Ratio	1.89
Consolidated Leverage Ratio, actual	2.75
Line of credit facility covenants tangible net worth description	Permit Tangible Net Worth as of the end of any fiscal quarter of the Company to be less than the sum of (i) $246.1 million plus (ii) 25% of the Company's Consolidated Net Income (as defined in the Amended Credit Facility) (but only if a positive number) for each fiscal quarter ended subsequent to December 31, 2011
Tangible Net Worth sum	294,200,000
Consolidated Net Income	25.00%
Percentage of net cash proceeds from issuance of capital stock	90.00%
Actual tangible net worth	386,400,000
Debt instrument description of variable rate basis	(i) LIBOR plus a defined margin, or (ii) the Agent bank’s prime rate (“base rate”) plus a margin. The applicable margin for each type of loan is measured based upon the Consolidated Leverage Ratio at the end of the prior fiscal quarter and ranges from 1.00% to 1.75% for LIBOR loans and 0% to 0.75% for base rate loans. In addition, the Company pays an unused commitment fee for the portion of the $420.0 million credit facility that is not used. These fees are based upon the Consolidated Leverage Ratio and range from 0.15% to 0.30%. As of December 31, 2014 and 2013, the applicable margins were 1.50% for LIBOR based loans, 0.50% for base rate loans and 0.25% for unused fees. Amounts borrowed under the Sweep Service Facility are based upon the Union Bank, N.A. base rate plus an applicable margin and an unused commitment fee for the portion of the $10.0 million facility not used.
Unused commitment fees	420,000,000
Amended Credit Facility [Member] \| Unused Fees [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end	0.25%		0.25%
Amended Credit Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Tangible Net Worth sum	246,100,000
Leverage ratio	0.15%
Amended Credit Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Leverage ratio	0.30%
Amended Credit Facility [Member] \| LIBOR Loans [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end	1.50%		1.50%
Amended Credit Facility [Member] \| LIBOR Loans [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Leverage ratio			1.00%
Amended Credit Facility [Member] \| LIBOR Loans [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Leverage ratio			1.75%
Amended Credit Facility [Member] \| Base Rate Loans [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end	0.50%		0.50%
Amended Credit Facility [Member] \| Base Rate Loans [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Leverage ratio			0.00%
Amended Credit Facility [Member] \| Base Rate Loans [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Leverage ratio			0.75%
Amended Credit Facility [Member] \| Sweep Service Facility [Member]
Debt Instrument [Line Items]
Credit facility, maximum	430,000,000
Revolving credit facility, available maximum borrowing capacity	227,500,000
Amount outstanding	202,500,000
Unused commitment fees	10,000,000
Letters of Credit [Member]
Debt Instrument [Line Items]
Credit facility, maximum	25,000,000
Letters of Credit [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Credit facility, maximum					5,000,000
Letters of Credit [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Credit facility, maximum					10,000,000
Letters of Credit [Member] \| Sweep Service Facility [Member]
Debt Instrument [Line Items]
Amount outstanding	2,500,000		4,000,000
Swingline Loans [Member]
Debt Instrument [Line Items]
Credit facility, maximum	$ 10,000,000