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Fair Value Measurements (Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2017 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2019 | Dec. 31, 2015 | Dec. 31, 2018 | Mar. 31, 2018 | Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Current maturities of long-term debt |  |  |  |  | $ 124.8 |  | $ 0.0 |  |
Long-term Debt, Excluding Current Maturities |  |  |  |  | $ 5,923.5 |  | 5,010.4 |  |
6.875% Senior notes due 2020 |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 6.875% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2020 |  |  |  |
6.875% Senior notes due 2020 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 6.875% |  |  |  |  |
Senior Note Maturity Year |  |  | 2020 | 2020 |  |  |  |  |
4.70% Senior notes due 2021 |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 4.70% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2021 |  |  |  |
4.70% Senior notes due 2021 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 4.70% |  |  |  |  |
Senior Note Maturity Year |  |  | 2021 | 2021 |  |  |  |  |
4.875%SeniorNotesDue2022Member [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 4.875% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2022 |  |  |  |
4.875%SeniorNotesDue2022Member [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 4.875% |  |  |  |
3.00% Exchangeable senior notes due 2024(2) |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 3.00% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2024 |  |  |  |
3.00% Exchangeable senior notes due 2024(2) | Convertible Debt [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 8.00% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2024 |  |  |  |
Carrying value of 2024 convertible notes |  |  |  |  | $ 838.3 |  | $ 836.3 |  |
4.50% Senior notes due 2024(1) |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 4.50% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2024 |  |  |  |
4.50% Senior notes due 2024(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 4.50% |  | 4.50% |  |  |
Senior Note Maturity Year |  |  |  |  |  | 2024 |  |  |
4.75%SeniorNotesDue2024Member [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 4.75% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2024 |  |  |  |
4.75%SeniorNotesDue2024Member [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 4.75% |  |  |  |
8.00% Senior notes due 2024(1) |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 8.00% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2024 |  |  |  |
8.00% Senior notes due 2024(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 8.00% | 8.00% |  | 8.00% |  |
Senior Note Maturity Year |  |  |  |  | 2024 |  |  |  |
5.20% Senior notes due 2025(1) |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 5.20% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2025 |  |  |  |
5.20% Senior notes due 2025(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 5.20% | 5.20% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2025 |  |  |  |
7.38%SeniorNotesDue2025 [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 7.375% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2025 |  |  |  |
7.38%SeniorNotesDue2025 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 7.375% |  |  |  |
7.75% Senior notes due 2026 |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 7.75% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2026 |  |  |  |
7.75% Senior notes due 2026 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  |  |  | 7.75% |
7.20% Senior notes due 2027 |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 7.20% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2027 |  |  |  |
7.20% Senior notes due 2027 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 7.20% |  |  | 7.20% |  |
7.875% Senior notes due 2040 |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 7.875% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2040 |  |  |  |
7.875% Senior notes due 2040 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 7.875% |  |  |  |  |
Senior Note Maturity Year |  |  |  | 2040 |  |  |  |  |
5.40%SeniorNotesDue2042 [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 5.40% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2042 |  |  |  |
5.40%SeniorNotesDue2042 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 5.40% |  |  |  |
5.75% Senior notes due 2044 |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 5.75% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2044 |  |  |  |
5.75% Senior notes due 2044 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  | 5.75% |  |  | 5.75%
Senior Note Maturity Year |  | 2044 |  |  |  | 2044 |  |  |
5.85%SeniorNotesDue2044 [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 5.85% |  |  |  |
Senior Note Maturity Year |  |  |  |  | 2044 |  |  |  |
5.85%SeniorNotesDue2044 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 5.85% |  |  |  |
Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | $ 6,048.3 |  | $ 5,010.4 |  |
Long-term Debt, Excluding Current Maturities |  |  |  |  | 5,923.5 |  | 5,010.4 |  |
Reported Value Measurement [Member] | 6.875% Senior notes due 2020 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 124.8 |  | 127.5 |  |
Reported Value Measurement [Member] | 4.70% Senior notes due 2021 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 113.2 |  | 112.7 |  |
Reported Value Measurement [Member] | 4.875%SeniorNotesDue2022Member [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 599.2 |  | 0.0 |  |
Reported Value Measurement [Member] | 3.00% Exchangeable senior notes due 2024(2) | Convertible Debt [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 699.0 |  | 666.8 |  |
Reported Value Measurement [Member] | 4.50% Senior notes due 2024(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 302.0 |  | 619.8 |  |
Reported Value Measurement [Member] | 4.75%SeniorNotesDue2024Member [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 276.5 |  | 0.0 |  |
Reported Value Measurement [Member] | 8.00% Senior notes due 2024(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 295.7 |  | 337.0 |  |
Reported Value Measurement [Member] | 5.20% Senior notes due 2025(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 331.7 |  | 664.4 |  |
Reported Value Measurement [Member] | 7.38%SeniorNotesDue2025 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 329.2 |  | 0.0 |  |
Reported Value Measurement [Member] | 7.75% Senior notes due 2026 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 987.1 |  | 985.0 |  |
Reported Value Measurement [Member] | 7.20% Senior notes due 2027 | Debentures Due 2027 |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 111.7 |  | 149.3 |  |
Reported Value Measurement [Member] | 7.875% Senior notes due 2040 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 373.3 |  | 375.0 |  |
Reported Value Measurement [Member] | 5.40%SeniorNotesDue2042 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 262.8 |  | 0.0 |  |
Reported Value Measurement [Member] | 5.75% Senior notes due 2044 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 973.3 |  | 972.9 |  |
Reported Value Measurement [Member] | 5.85%SeniorNotesDue2044 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  |  | 268.8 |  | 0.0 |  |
Estimate of Fair Value Measurement [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 3,874.1 |  | 3,545.8 |  |
Current maturities of long-term debt |  |  |  |  | 0.0 |  | 0.0 |  |
Long-term Debt, Fair Value |  |  |  |  | 3,874.1 |  | 3,545.8 |  |
Estimate of Fair Value Measurement [Member] | 6.875% Senior notes due 2020 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 117.3 |  | 121.6 |  |
Estimate of Fair Value Measurement [Member] | 4.70% Senior notes due 2021 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 95.5 |  | 101.8 |  |
Estimate of Fair Value Measurement [Member] | 4.875%SeniorNotesDue2022Member [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 460.5 |  | 0.0 |  |
Estimate of Fair Value Measurement [Member] | 3.00% Exchangeable senior notes due 2024(2) | Convertible Debt [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure | [1] |  |  |  | 607.4 |  | 575.5 |  |
Estimate of Fair Value Measurement [Member] | 4.50% Senior notes due 2024(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 167.2 |  | 405.2 |  |
Estimate of Fair Value Measurement [Member] | 4.75%SeniorNotesDue2024Member [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 201.4 |  | 0.0 |  |
Estimate of Fair Value Measurement [Member] | 8.00% Senior notes due 2024(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 181.7 |  | 273.7 |  |
Estimate of Fair Value Measurement [Member] | 5.20% Senior notes due 2025(1) | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 186.7 |  | 443.9 |  |
Estimate of Fair Value Measurement [Member] | 7.38%SeniorNotesDue2025 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 218.6 |  | 0.0 |  |
Estimate of Fair Value Measurement [Member] | 7.75% Senior notes due 2026 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 575.1 |  | 725.5 |  |
Estimate of Fair Value Measurement [Member] | 7.20% Senior notes due 2027 | Debentures Due 2027 |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 70.0 |  | 109.1 |  |
Estimate of Fair Value Measurement [Member] | 7.875% Senior notes due 2040 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 153.5 |  | 223.2 |  |
Estimate of Fair Value Measurement [Member] | 5.40%SeniorNotesDue2042 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 194.4 |  | 0.0 |  |
Estimate of Fair Value Measurement [Member] | 5.75% Senior notes due 2044 | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | 450.0 |  | 566.3 |  |
Estimate of Fair Value Measurement [Member] | 5.85%SeniorNotesDue2044 [Member] | Senior Notes [Member] |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, Fair Value Disclosure |  |  |  |  | $ 194.8 |  | $ 0.0 |  |
[1] | Our 2024 Convertible Notes were issued with a conversion feature. The 2024 Convertible Notes were separated into their liability and equity components on our consolidated balance sheet. The equity component was initially recorded to additional paid-in capital and as a debt discount that will be amortized to interest expense over the life of the instrument. Excluding the unamortized discount, the carrying value of the 2024 Convertible Notes was $838.3 million and $836.3 million as of December 31, 2019 and 2018, respectively. See "Note 7 - Debt" for additional information on this issuance.

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