Summary of Estimated Fair Values of Investments and Remaining Unfunded Commitments for Each Major Category of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Carrying Amount	$ 870,972
Fair Value	904,157
Unfunded Commitments	498,357
Non-marketable securities | Fair value accounting | Venture capital and private equity fund investments
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Carrying Amount	658,409
Fair Value	658,409
Unfunded Commitments	430,798
Non-marketable securities | Equity method accounting | Other investments
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Carrying Amount	54,288
Fair Value	55,715
Unfunded Commitments	6,750
Non-marketable securities | Cost method accounting | Venture capital and private equity fund investments
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Carrying Amount	158,275
Fair Value	190,033
Unfunded Commitments	$ 60,809