| **Cash and Marketable Securities - Restricted Marketable Securities (Details) - USD ($) $ in Millions** | Dec. 31, 2018 | Dec. 31, 2017 |
|---------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Summary of Restricted Marketable Securities**                                                         |          |          |
| Restricted marketable securities, cost                                                                  |          | $ 64.0        |
| Restricted marketable securities, gross unrealized gains                                                |          | 0.3           |
| Restricted marketable securities, gross unrealized losses                                               |          | (2.3)         |
| Restricted marketable securities, fair value                                                            | $ 62.0        | $ 62.0        |

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| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
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| - Definition
The carrying amount of securities categorized neither as held-to-maturity nor trading which serve as security for borrowings or other arrangements.
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