Marketable Securities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt Securities
Schedule Of Marketable Securities [Line Items]
Realized gains	$ 2	$ 86	$ 3,450
Seed Capital Investments
Schedule Of Marketable Securities [Line Items]
Realized gains	(85)	936	1,653
Debt Securities Carried by EGL
Schedule Of Marketable Securities [Line Items]
Marketable securities, gain (loss)	(674)	(460)	481
Mutual Funds
Schedule Of Marketable Securities [Line Items]
Marketable securities, gain (loss)	$ 1,025	$ (412)

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

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This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

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