Employee Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Period of service before an employee is eligible for contribution	1 year
Common stock authorized for issuance but not yet issued for ESPP	248
Common stock authorized and reserved for ESPP	250
ESPP employee contribution, percentage	15.00%cmg_EsppPercentOfBaseEarnings
Shares issued under ESPP	1	1	1
Fair value of investments in rabbi trust	$ 16,147us-gaap_TradingSecurities	$ 13,397us-gaap_TradingSecurities
Unrealized gains (losses) on investments held in rabbi trust	184	722	240
Deferred Plan [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Employer matching contribution, percentage on first 3%	100.00%
Employer matching contribution, percentage on next 2%	50.00%
Percentage of employees' gross pay for which the employer contributes a matching contribution of 100% under the Deferred Plan	3.00%
Percentage of employees' gross pay for which the employer contributes a matching contribution of 50% under the Deferred Plan	2.00%
Deferred compensation matching contributions	536	201	213
Total liabilities under Deferred Plan	16,147	13,397
401(k) Plan [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Employer matching contribution, percentage on first 3%	100.00%
Employer matching contribution, percentage on next 2%	50.00%
Percentage of employees' gross pay for which the employer contributes a matching contribution of 100%	3.00%
Percentage of employees' gross pay for which the employer contributes a matching contribution of 50%	2.00%
Period of service before an employee is eligible for contribution	1 year
Company matching contributions	$ 3,881	$ 2,644	$ 2,431

Common stock reserved for issuance under the Company's ESPP.

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Common stock authorized for issuance but not yet issued for ESPP

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- Definition

Deferred Compensation Arrangement With Individual Employer Contribution Percentage On First 3 Percent

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Deferred Compensation Arrangement With Individual Employer Contribution Percentage On Next 2 Percent

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- Definition

Deferred compensation arrangement with individual, employer matching contribution of 100 percent

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Deferred compensation arrangement with individual, employer matching contribution of 50 percent

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- Definition

Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay, at Which Employer Matches 100 Percent

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- Definition

Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay, at Which Employer Matches 50 Percent

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- Definition

Defined Contribution Plan, Employer Matching Contribution, Percent of Match On First 3%

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Defined Contribution Plan, Employer Matching Contribution, Percent of Match On Next 2 Percent

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- Definition

The maximum percent an employee may contribute after one year of service with the Company.

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- Definition

Eligibility period to contribute to the ESPP.

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Eligibility period to receive matching contributions

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- Definition

The amount of payment into the fund established for purposes of making future disbursements to the individual in accordance with the terms and understanding of the deferred compensation arrangement.

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No definition available.

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- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable beyond one year (or the operating cycle, if longer).

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- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

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- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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- Definition

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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Amount of unrealized holding gain (loss) recognized in the income statement for investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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-Publisher FASB

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-SubTopic 10

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-Subparagraph (SX 210.5-03.7(c))

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