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Long-Term Debt (Narrative) (Details) - USD ($) $ in Millions				1 Months Ended		3 Months Ended	12 Months Ended
	Aug. 18, 2020	Apr. 13, 2020	Aug. 12, 2019	Sep. 30, 2020	Aug. 31, 2020	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 01, 2019
Debt Instrument [Line Items]
Debt Instrument Tendered					$ 969.3
Loss on early extinguishment of long-term debt						$ 243.4	$ (243.5)	$ 0.0	$ (45.5)
Debt Instrument, Convertible, Conversion Ratio							62.50%
Security interest and other subset of asset							$ 150.0
Asset sale covenant percentage of consolidated total assets							10.00%
Cross default provision on default relating to any indebtedness							$ 50.0
Percentage of additional subset of assets capped							10.00%
Working Capital Adjustment				$ 5.3		$ 5.3
Senior Notes
Debt Instrument [Line Items]
Face amount of notes		$ 1,000.0
Columbia Of Massachusetts | 6.30% Notes Due 2028
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate							6.37%	630.00%
Maximum
Debt Instrument [Line Items]
Cross default provision on default relating to any indebtedness							$ 50.0
Minimum
Debt Instrument [Line Items]
Cross default provision on default relating to any indebtedness							$ 5.0
5.85% Notes Due 2019
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount										$ 41.0
Interest rate on debt										5.85%
2.95% Notes Due 2029
Debt Instrument [Line Items]
Interest rate on debt			2.95%
2.95% Notes Due 2029 | Senior Notes
Debt Instrument [Line Items]
Face amount of notes			$ 750.0
Proceeds from Debt, Net of Issuance Costs			$ 742.4
3.65% Notes Due 2023
Debt Instrument [Line Items]
Interest rate on debt				3.65%	3.65%	3.65%
0.95% Notes Due 2025
Debt Instrument [Line Items]
Interest rate on debt	0.95%
0.95% Notes Due 2025 | Senior Notes
Debt Instrument [Line Items]
Face amount of notes	$ 1,250.0
3.60% Notes Due 2030
Debt Instrument [Line Items]
Interest rate on debt		3.60%
3.60% Notes Due 2030 | Senior Notes
Debt Instrument [Line Items]
Proceeds from Debt, Net of Issuance Costs		$ 987.8
1.70% Notes Due 2031
Debt Instrument [Line Items]
Interest rate on debt	1.70%
1.70% Notes Due 2031 | Senior Notes
Debt Instrument [Line Items]
Face amount of notes	$ 750.0
0.95% Notes Due 2025 and 1.70% Notes Due 2031 [Member] | Senior Notes
Debt Instrument [Line Items]
Proceeds from Debt, Net of Issuance Costs	$ 1,980.4
4.45% Notes Due 2021
Debt Instrument [Line Items]
Interest rate on debt				4.45%	4.45%	4.45%
2.65% Notes Due 2022
Debt Instrument [Line Items]
Interest rate on debt				2.65%	2.65%	2.65%
3.85% Notes Due 2023
Debt Instrument [Line Items]
Interest rate on debt				3.85%	3.85%	3.85%
6.25% Notes Due 2040
Debt Instrument [Line Items]
Interest rate on debt					6.25%
5.95% Notes Due 2041
Debt Instrument [Line Items]
Interest rate on debt					5.95%
5.89% Notes Due 2025
Debt Instrument [Line Items]
Interest rate on debt				5.89%		5.89%
Senior Note Redeemed August & September 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount				$ 609.3		$ 609.3
Loss on early extinguishment of long-term debt				231.8
Note Redeemed September 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount				25.0		$ 25.0
Loss on early extinguishment of long-term debt				$ 11.7
6.26% Notes Due 2028 [Member]
Debt Instrument [Line Items]
Interest rate on debt				6.26%		6.26%
Revolving Credit Facility | Maximum
Debt Instrument [Line Items]
Debt Instrument, Convertible, Conversion Ratio							70.00%
Private Placement [Member] | Maximum
Debt Instrument [Line Items]
Debt Instrument, Convertible, Conversion Ratio							75.00%

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