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Long-Term Debt Interest Expense and Investment Income (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Debt Disclosure [Abstract]      
Interest Expense $ 99.2 $ 99.9 $ 115.9
Interest Costs Capitalized $ 4.5 $ 5.8 $ 6.6
Long-term Debt, Weighted Average Interest Rate 6.50% 6.40% 6.30%
Investment Income, Interest $ 2.6 $ 2.1 $ 0.6