Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=mpg_ShareBasedCompensationAwardTrancheSevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=mpg_ShareBasedCompensationAwardTrancheSixMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=mpg_ShareBasedCompensationAwardTrancheTwoOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=mpg_SharebasedCompensationAwardTrancheFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=mpg_SharebasedCompensationAwardTrancheFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock-based Compensation - Summary of Terms of Stock Options (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options	2,998	3,400	1,938
Weighted Average Exercise Price	$ 12.46	$ 10.82	$ 10.60
Contractual Terms	6 years 2 months 12 days	7 years 4 months 24 days	8 years 2 months 12 days
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options	7,621
Employee Stock Option [Member] \| 1/3rd per year on grant-date anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting Terms	1/3rd per year on grant-date anniversary
Number of Options	389
Weighted Average Exercise Price	$ 15.81
Contractual Terms	10 years
Employee Stock Option [Member] \| 1/3rd per year on grant-date anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting Terms	1/3rd per year on grant-date anniversary
Number of Options	352
Weighted Average Exercise Price	$ 15.08
Contractual Terms	10 years
Employee Stock Option [Member] \| 1/3rd per year on grant-date anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting Terms	1/3rd per year on grant-date anniversary
Number of Options	438
Weighted Average Exercise Price	$ 18.90
Contractual Terms	10 years
Employee Stock Option [Member] \| Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting Terms	100% upon grant
Number of Options	635
Weighted Average Exercise Price	$ 20.00
Contractual Terms	10 years
Employee Stock Option [Member] \| Share Based Compensation Award Tranche Two Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting Terms	1/3rd per year on grant-date anniversary
Number of Options	560
Weighted Average Exercise Price	$ 20.00
Contractual Terms	10 years
Employee Stock Option [Member] \| Share Based Compensation Award Tranche Three
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting Terms	1/5th upon grant and 1/5th on December 6th of following 4 years
Number of Options	357
Weighted Average Exercise Price	$ 20.00
Contractual Terms	10 years
Employee Stock Option [Member] \| Sharebased Compensation Award Tranche Five
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting Terms	1/5th per year on the anniversary of the original grant date
Number of Options	4,405
Weighted Average Exercise Price	$ 6.32
Contractual Terms	10 years
Employee Stock Option [Member] \| Sharebased Compensation Award Tranche Four
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting Terms	100% on the seventh anniversary of the original grant date
Number of Options	485
Weighted Average Exercise Price	$ 18.66
Contractual Terms	10 years