Summary of Significant Accounting Policies - Summary of Ownership Interests in Investments Held Under Fair Value Accounting (Details) - Non-marketable securities - Consolidated venture capital and private equity fund investments - Fair value accounting	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Strategic Investors Fund, LP
Schedule of Investments [Line Items]
Company Direct and Indirect Ownership in Limited Partnership	12.60%	12.60%
Capital Preferred Return Fund, LP
Schedule of Investments [Line Items]
Company Direct and Indirect Ownership in Limited Partnership	20.00%	20.00%
Growth Partners, LP
Schedule of Investments [Line Items]
Company Direct and Indirect Ownership in Limited Partnership	33.00%	33.00%
CP I, LP
Schedule of Investments [Line Items]
Company Direct and Indirect Ownership in Limited Partnership	10.70%	10.70%