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Investment Securities (Significant Unobservable Inputs) (Details) $ in Millions | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 input
BBB | Debt securities issued by corporations | Broker Pricing |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Assets, Fair Value Disclosure | $ 5.6 |  |
Valuation techniques | 133.792 |  |
NR | Private equity funds |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Time until expiration of security | 4 years | 4 years |
Volatility rate | 50.00% | 50.00% |
Risk free rate | 1.00% | 1.00% |
NR | Private equity funds | Discounted cash flows |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Assets, Fair Value Disclosure | $ 22.1 | $ 22.1 |
Valuation techniques | 0.25 | 0.25 |
NR | Private equity funds | Option pricing method |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Assets, Fair Value Disclosure | $ 10.4 | $ 9.3 |
Valuation techniques | 0.21 | 0.21 |
NR | Convertible Preferred Stock | Multiple of EBITDA |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Assets, Fair Value Disclosure | $ 0.2 | $ 3.6 |
Valuation techniques | 6 | 6.0 |
Unobservable inputs | input |  |  | 0
Investment A | NR | Private equity funds | Recent transaction |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Assets, Fair Value Disclosure | $ 21.0 | $ 21.0 |
Valuation techniques | 1 | 1.00 |
Investment B | NR | Private equity funds | Recent transaction |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Assets, Fair Value Disclosure | $ 3.6 | $ 3.2 |
Valuation techniques | 2.521 | 2.52 |
Investment E | NR | Convertible Preferred Stock | Recent transaction |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Assets, Fair Value Disclosure | $ 27.0 | $ 27.0 |
Valuation techniques | 3.83 | 3.83 |

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