Investments (Schedule of Fair Value of Equity Investments Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity investments trading	$ 387,462	$ 388,254
Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity investments trading	253,905	253,543
Communications and technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity investments trading	51,400	49,526
Industrial, utilities and energy
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity investments trading	32,790	34,325
Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity investments trading	23,886	24,779
Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity investments trading	21,042	21,364
Basic materials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity investments trading	$ 4,439	$ 4,717