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Dispositions (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Dividend of subsidiary $ 35us-gaap_DividendsCommonStockPaidinkind $ 0us-gaap_DividendsCommonStockPaidinkind $ 0us-gaap_DividendsCommonStockPaidinkind
New England Securities Corporation [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Dividend of subsidiary 35us-gaap_DividendsCommonStockPaidinkind
/ dei_LegalEntityAxis
= mlic_NewEnglandSecuritiesCorporationMember		    
Retained Earnings      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Dividend of subsidiary 35us-gaap_DividendsCommonStockPaidinkind
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember		    
Retained Earnings | New England Securities Corporation [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Dividend of subsidiary $ 35us-gaap_DividendsCommonStockPaidinkind
/ dei_LegalEntityAxis
= mlic_NewEnglandSecuritiesCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember