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

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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