Investment Securities (Significant Unobservable Inputs) (Details) - NR	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Kudu Investments | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 142,200,000
MediaAlpha
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	114,700,000
DavidShield & PassportCard | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	75,000,000.0	$ 75,000,000.0
Elementum Holdings, L.P. | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	55,100,000
Private Debt Instrument | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	21,700,000	10,000,000.0
durchblicker | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	15,500,000	15,500,000
Vidado (f.k.a. Captricity, Inc.) | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	14,500,000
Captricity | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		14,500,000
Galvanic Applied Sciences | Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	2,200,000	3,100,000
Compare.com | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 16,900,000
Compare.com | Estimated Net Realizable Value
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 1,900,000
Unit price | MediaAlpha
Quantitative information for Level 3 Fair Value Measurements Assets
Share Price | $ / shares	$ 208.98
Discount rate | DavidShield & PassportCard
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	18.00%	18.00%
Discount rate | Private Debt Instrument
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate		9.23%
Discount rate | durchblicker
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%	23.00%
Discount rate | Vidado (f.k.a. Captricity, Inc.)
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%
Discount rate | Captricity
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate		23.00%
Discount rate | Compare.com
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate		22.00%
Revenue exit multiple | DavidShield & PassportCard
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	1.00	1.00
Revenue exit multiple | durchblicker
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	2.25	2.25
Revenue exit multiple | Captricity
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	3.75	3.75
Revenue exit multiple | Compare.com
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input		2.75
Transaction price | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	5510000000.00%
EBITDA multiple | Galvanic Applied Sciences
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	600.00%	600.00%
Discount to Book Value | Compare.com
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	94.00%
Minimum | Discount rate | Kudu Investments
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	15.30%
Minimum | Discount rate | Private Debt Instrument
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	6.56%
Minimum | Revenue exit multiple | Kudu Investments
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	6.60%
Maximum | Discount rate | Kudu Investments
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	19.60%
Maximum | Discount rate | Private Debt Instrument
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	8.71%
Maximum | Revenue exit multiple | Kudu Investments
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	12.20%