Long-Term Debt - Interest Expense and Interest Income Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Interest Expense	$ 217.3	$ 216.3	$ 195.6
Interest Costs Capitalized	$ 15.5	$ 28.4	$ 18.3
Long-term Debt, Weighted Average Interest Rate, at Point in Time	5.80%	5.90%	5.70%
Investment Income, Interest	$ 5.8	$ 2.0	$ 2.9
Unsecured Debt [Member] | 6.25% Notes due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 700.0	700.0		$ 700.0
Debt Instrument, Interest Rate, Stated Percentage	6.25%			6.25%
Unsecured Debt [Member] | 6.50% Notes [Member]
Debt Instrument [Line Items]
Long-term debt	$ 500.0	$ 500.0		$ 500.0
Debt Instrument, Interest Rate, Stated Percentage	6.50%			6.50%