Investments (Schedule of Fair Value of Equity Investments Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Equity investments, trading	$ 310,252	$ 388,254
Financials
Investment [Line Items]
Equity investments, trading	200,357	253,543
Communications and technology
Investment [Line Items]
Equity investments, trading	42,333	49,526
Industrial, utilities and energy
Investment [Line Items]
Equity investments, trading	24,520	34,325
Consumer
Investment [Line Items]
Equity investments, trading	20,639	24,779
Healthcare
Investment [Line Items]
Equity investments, trading	18,925	21,364
Basic materials
Investment [Line Items]
Equity investments, trading	$ 3,478	$ 4,717