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Debt (Narrative - Corporate Activities) (Details) - Corporate Activities [Member] - Convertible Notes [Member] - USD ($) | 1 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Aug. 31, 2016 | Sep. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019
4.00% Convertible Notes Due 2024 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt instrument, face amount |  | $ 115,000,000.0 | $ 115,000,000.0 |  |
Interest rate, stated percentage |  |  | 4.00% | 4.00% | 4.00%
Debt Instrument, Maturity Date |  | Jul. 01, 2024 |  |  |
Common stock for conversion, shares |  |  | 64.1540 |  |
Debt Instrument Convertible Conversion Price Benchmark1 |  |  | $ 1,000 |  |
Debt conversion price |  |  | $ 15.59 |  |
Debt Conversion, Sale Price Of Common Stock Percent, Minimum |  |  | 140.00% |  |
Debt Instrument, Redemption Price, Percentage |  |  | 100.00% |  |
4.125% Convertible Notes Due 2022 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt instrument, face amount | $ 170,000,000.0 | $ 170,000,000.0 | $ 170,000,000.0 |  |
Interest rate, stated percentage | 4.125% | 4.125% |  |  |
Debt conversion, principal amounts for integral multiples | $ 1,000 |  |  |  |
Common stock for conversion, shares | 35.7143 |  |  |  |
Debt conversion price | $ 28.00 |  |  |  |
Conversion price percentage | 140.00% |  |  |  |
Principal amount of notes, percentage | 100.00% | 100.00% |  |  |

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