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Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Feb. 29, 2016 | Sep. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Debt Instrument [Line Items] |  |  |  |
Shares repurchased, weighted average price per share |  | $ 8.03 |  |
Shares repurchased, value |  | $ 147.1 |  |
Shareholder rights accompanying each outstanding share of the company's common stock (in number of Rights) |  |  | 1 |
Common stock, beneficial ownership threshold to be considered an Acquiring Person (in hundredths) |  |  | 5.00% |
Purchase price (in dollars per share) |  |  | $ 45 |
Purchase price (in dollars per one-tenth share) |  |  | 4.50 |
Redemption price (in dollars per Right) |  |  | $ 0.001 |
Rights |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Period after person becomes acquiring person |  |  | 10 days |
Common stock purchased per right |  |  | 0.1 |
5% Convertible Senior Notes due 2017 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Extinguishment of debt, amount |  |  | $ 188.5 | $ 11.5
Stated interest rate (in hundredths) |  |  | 5.00% |
5% Convertible Senior Notes due 2017 | 2% Convertible Senior Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Extinguishment of debt, amount |  | $ 292.4 |  |
Stated interest rate (in hundredths) |  |  | 2.00% |
Repayments of debt (in shares) |  | 18,300,000 |  |
2% Convertible Senior Notes due 2020 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (in hundredths) |  |  | 2.00% |
Convertible Junior Subordinated Debentures Due 2063 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (in hundredths) |  |  | 9.00% |
9% Convertible Junior Subordinated Debentures due 2063 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Extinguishment of debt, amount | $ 132.7 |  |  |
Stated interest rate (in hundredths) |  |  | 9.00% |
Reacquisition of convertible junior subordinated debentures-equity component |  |  | $ 6.3 |

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