Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total	$ 2,785.0	$ 2,735.0
Unamortized debt issuance costs	(8.0)	(6.0)
Unamortized discount, net	(17.8)	(19.4)
Long-term debt including current portion	2,759.2	2,709.6
Current portion of long-term debt	0.0	(250.0)
Total long-term debt	2,759.2	2,459.6
Long-term debt outstanding maturities
2020	0.0
2021	300.0
2022	0.0
2023	0.0
2024	300.0
Thereafter	2,185.0
Total	$ 2,785.0	2,735.0
Debentures (unsecured), 4.25% due 2019
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.25%
Unsecured Debt	$ 0.0	250.0
Repayments of Unsecured Debt	$ 250.0
Debentures (unsecured), 2.95% due 2021
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.95%
Unsecured Debt	$ 300.0	300.0
Debentures (unsecured), 2.05% due 2024
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.05%
Unsecured Debt	$ 300.0	0.0
Proceeds from issuance of debt	$ 300.0
Debentures (unsecured), 3.10% due 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.10%
Unsecured Debt	$ 250.0	250.0
Debentures (unsecured), 6.50% due 2028
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.50%
Unsecured Debt	$ 150.0	150.0
Debentures (unsecured), 5.625% due 2033
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.625%
Unsecured Debt	$ 335.0	335.0
Debentures (unsecured), 5.70% due 2036
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.70%
Unsecured Debt	$ 300.0	300.0
Debentures (unsecured), 3.65% due 2042
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.65%
Unsecured Debt	$ 250.0	250.0
Debentures (unsecured), 4.25% due 2044
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.25%
Unsecured Debt	$ 250.0	250.0
Debentures (unsecured), 4.30% due 2045
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.30%
Unsecured Debt	$ 250.0	250.0
Debentures (unsecured), 4.30% due 2048
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.30%
Unsecured Debt	$ 300.0	300.0
Debentures (unsecured), 6.875% due 2095
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.875%
Unsecured Debt	$ 100.0	$ 100.0