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Fair Value Measurements - Narrative (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
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Sep. 30, 2017 USD ($) quote / Investment | Dec. 31, 2016 USD ($) quote / Investment | Sep. 30, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2017 USD ($) quote / Investment | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($) quote / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Quotes per fixed income investment | quote / Investment | 2.0 | 2.1 |  |  |  | 2.0 |  | 2.1
Quotes per equity investment | quote / Investment | 4.0 | 3.9 |  |  |  | 4.0 |  | 3.9
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 |  |  |  |  |  |
Non-U.S. government |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Level 1 to Level 2 transfers, amount |  | $ 83.9 |  |  |  |  |  | $ 83.9
Fair Value, Measurements, Recurring | Level 3 | Silverton |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  | $ 89.3
Fair Value, Measurements, Recurring | Level 3 | Silverton 2015 |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Amount of Level 3 liabilities settled |  |  | $ 0.3 |  |  |  | $ 89.3 |
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 |  |

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