Investment Securities - Summary of Venture Capital and Private Equity Fund Investments Held by Consolidated Funds and Percentage Ownership (Details) - Fair value accounting - Consolidated venture capital and private equity fund investments - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Nonmarketable securities	$ 108,404	$ 118,333
Strategic Investors Fund, LP
Investment Holdings [Line Items]
Nonmarketable securities	8,618	12,452
Capital Preferred Return Fund, LP
Investment Holdings [Line Items]
Nonmarketable securities	51,219	53,957
Growth Partners, LP
Investment Holdings [Line Items]
Nonmarketable securities	47,532	50,845
CP I, LP
Investment Holdings [Line Items]
Nonmarketable securities	$ 1,035	$ 1,079
Non-marketable securities | Strategic Investors Fund, LP
Investment Holdings [Line Items]
Percentage of ownership	12.60%	12.60%
Non-marketable securities | Capital Preferred Return Fund, LP
Investment Holdings [Line Items]
Percentage of ownership	20.00%	20.00%
Non-marketable securities | Growth Partners, LP
Investment Holdings [Line Items]
Percentage of ownership	33.00%	33.00%
Non-marketable securities | CP I, LP
Investment Holdings [Line Items]
Percentage of ownership	10.70%	10.70%