Investments - Components of Net Realized and Unrealized Gains/(Losses) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Gross realized gains on available-for-sale securities	$ 3,735	$ 201	$ 5,209	$ 769
Gross realized losses on available-for-sale securities	(27)	(19)	(450)	(329)
Net realized gains on trading securities	3,824	1,317	12,684	4,463
Net unrealized gains (losses) on trading securities	13,059	(10,479)	25,382	(5,251)
Net unrealized gains on other investments	7,689	468	12,528	7,331
Net realized and unrealized gains (losses)	28,280	(8,512)	55,353	6,983
Proceeds from sales and maturities of available-for-sale securities	$ 112,928	$ 70,583	$ 296,537	$ 332,560

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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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of trading securities held at each balance sheet date, that was included in earnings for the period. Trading securities are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) and the change in net unrealized holding gain (loss) is included in earnings for the period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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