Investment Securities (Significant Unobservable Inputs) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 input
Standard & Poor's, A- Rating | Non-Agency Residential Mortgage | Broker Pricing
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 2.0
Valuation techniques	100.03125
Standard & Poor's, AAA Rating | Non-Agency Residential Mortgage | Broker Pricing
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 7.7
Valuation techniques	102.03125
BBB | Debt securities issued by corporations | Broker Pricing
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 4.4
Valuation techniques	127.45687
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	$ 10.0	$ 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	$ 1.3	$ 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	$ 3.4	$ 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