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Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 segment | Mar. 31, 2017 USD ($) | Jan. 01, 2018 USD ($)
Number of reportable segments | segment | 1 |  |
Interest in subsidiary (less than given percentage) | 100.00% |  |
Accounting Standards Update 2016-01 |  |  |
Cumulative effect of new accounting principle |  |  | $ 1.5
Accounting Standards Update 2016-01 | Retained Earnings |  |  |
Cumulative effect of new accounting principle |  |  | 0.0
Accounting Standards Update 2016-16 | Net deferred tax asset |  |  |
Cumulative effect of new accounting principle |  |  | 500.0
Accounting Standards Update 2016-16 | Retained Earnings |  |  |
Cumulative effect of new accounting principle |  |  | $ 500.0
Accounting Standards Updated 2017-07 | Other income (expense) |  |  |
Prior period reclassification adjustment |  | $ 0.4 |

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