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Fair Value Measurements - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 USD ($) quote / Investment | Dec. 31, 2016 USD ($) quote / Investment | Mar. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 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 | 2.3 | 2.1 |  |  |  | 2.1
Quotes per equity investment | quote / Investment | 4.0 | 3.9 |  |  |  | 3.9
Silverton 2015 |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Notes Issued |  |  |  |  | $ 85.0 |
Silverton 2016 |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Notes Issued |  |  |  | $ 125.0 |  |
Aspen Holdings and Third Parties | Silverton 2017 |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Notes Issued |  | $ 130.0 |  |  |  |
Third party | Silverton 2015 |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Notes Issued |  |  |  |  | $ 70.0 |
Third party | Silverton 2016 |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Notes Issued |  |  |  | $ 100.0 |  |
Third party | Silverton 2017 |  |  |  |  |  |
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 2015 |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Amount of Level 3 liabilities settled |  |  | $ 87.4 |  |  |
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 | $ 111.2 |  |  |  |  |
Third party | Fair Value, Measurements, Recurring | Level 3 |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Repayments of debt |  |  |  |  |  | $ 89.3

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