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Stockholder's Equity (Narrative) (Details) (USD $)
0 Months Ended 12 Months Ended
Aug. 06, 2014
Dec. 03, 2012
Dec. 31, 2014
item
May 20, 2014
Oct. 31, 2012
Shareholders Equity [Line Items]          
Special dividend, per outstanding common share   $ 2.50us-gaap_CommonStockDividendsPerShareDeclared      
Special dividend   $ 486,141,000us-gaap_PaymentsOfDividends      
Special dividends record date     Nov. 16, 2012    
Share buyback program, authorization 500,000,000us-gaap_StockRepurchaseProgramAuthorizedAmount1     125,000,000us-gaap_StockRepurchaseProgramAuthorizedAmount1  
Number of share buyback programs     2mur_NumberOfShareBuybackPrograms    
Shares repurchased, shares 0us-gaap_TreasuryStockSharesAcquired        
Maximum [Member]          
Shareholders Equity [Line Items]          
Share buyback program, authorization         $ 1,000,000,000.0us-gaap_StockRepurchaseProgramAuthorizedAmount1
/ us-gaap_RangeAxis
= us-gaap_MaximumMember