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Fair Value Measurements - Narrative (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 USD ($) quote / Investment | Sep. 30, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2018 USD ($) quote / Investment | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) quote / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Quotes per fixed income investment | quote / Investment | 2.2 |  |  |  |  | 2.2 |  | 2.0
Silverton 2015 |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Notes Issued |  |  |  |  | $ 85.0 |  |  |
Silverton 2015, Third-party Funded |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Notes Issued |  |  |  |  | $ 70.0 |  |  |
Silverton 2016 |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Notes Issued |  |  |  | $ 125.0 |  |  |  |
Silverton 2016, Third-party Funded |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Notes Issued |  |  |  | $ 100.0 |  |  |  |
Silverton 2017 |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Notes Issued |  |  | $ 130.0 |  |  |  |  |
Silverton 2017, Third-party Funded |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Notes Issued |  |  | $ 105.0 |  |  |  |  |
Fair Value, Measurements, Recurring | Level 3 | Silverton |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  | $ 115.6
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016 |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Amount of Level 3 liabilities settled |  | $ 1.4 |  |  |  |  | $ 115.9 |
Fair Value, Measurements, Recurring | Level 3 | Silverton 2017 |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Amount of Level 3 liabilities settled | $ 14.7 |  |  |  |  | $ 78.4 |  |

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