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Senior Notes (Narrative) (Details) - Senior Notes [Member] - 4.868% Senior notes due 6/1/2044 [Member] - USD ($)
3 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Date Issued Jun. 05, 2014  
Principal amounts $ 400,000,000  
Debt maturity term 30 years  
Debt instrument, stated interest rate 4.868% 4.868%
Date due Jun. 01, 2044