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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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