Total Capital Commitments, Unfunded Capital Commitments, and Our Ownership in Each Fund (Footnote Information) (Detail) - Venture capital and private equity fund investments - Investment	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
CP II, L.P. | Fair value accounting | Direct ownership interest
Venture Capital and Private Equity Fund Investments [Line Items]
Company Direct and Indirect Ownership in Limited Partnership	1.30%
CP II, L.P. | Fair value accounting | Indirect ownership interest
Venture Capital and Private Equity Fund Investments [Line Items]
Company Direct and Indirect Ownership in Limited Partnership	3.80%
Other private equity fund | Fair value accounting
Venture Capital and Private Equity Fund Investments [Line Items]
Company Direct and Indirect Ownership in Limited Partnership	0.00%	58.20%
Other Fund Investments
Venture Capital and Private Equity Fund Investments [Line Items]
Number of other funds with investment commitments	241
Upper Limit | Other Fund Investments
Venture Capital and Private Equity Fund Investments [Line Items]
Company Direct and Indirect Ownership in Limited Partnership	5.00%