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Fair Value Measurements (Narrative) (Details) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Fair Value [Line Items]          
Notional amount $ 2,500,000,000   $ 2,500,000,000    
Fair value of interest rate swaps 27,400,000   27,400,000   76,900,000
Interest rate swap maturity date September 30, 2013   September 30, 2013   September 30, 2013
Proceeds from sale of investment securities     135,496,000 0  
Gain on marketable securities 130,898,000 0 130,898,000 0  
Level 2 [Member]
         
Fair Value [Line Items]          
Fair value level classification $ 27,400,000   $ 27,400,000   $ 76,900,000