Investment in Securities - Additional Information (Detail) (JPY ¥) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended		9 Months Ended	12 Months Ended
Dec. 31, 2011 Investment	Dec. 31, 2010	Dec. 31, 2011 Investment	Dec. 31, 2010	Mar. 31, 2011 Investment	Dec. 31, 2011 CMBS and RMBS in the U.S., and other asset-backed securities	Mar. 31, 2011 CMBS and RMBS in the U.S., and other asset-backed securities
Schedule of Investments [Line Items]
Non-credit components of other-than-temporary impairments						¥ 1,048	¥ 392
Approximate number of investment securities in an unrealized loss position	326		326		449
Aggregate carrying amount of other securities accounted for under the cost method	94,391		94,391		67,366
Aggregate carrying amount of other securities not evaluated for impairment	93,253		93,253		63,590
Interest income on investment securities	¥ 3,756	¥ 4,183	¥ 11,235	¥ 13,022

This element represents the aggregate carrying amount of all cost-method investments held by the Company. Generally, such investments do not have readily determinable fair values and are not required to be accounted for by the equity method. The carrying amount of such securities equates to cost, adjusted for other-than-temporary impairment. The cost method and lower-of-cost or market, an adaptation of the cost method, is generally followed for most investments in noncontrolled corporations, in some corporate joint ventures, and to a lesser extent in unconsolidated subsidiaries, particularly foreign.

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This element represents the aggregate carrying amount of all cost-method investments held by the Entity which was not evaluated for impairment. Because the fair value of cost-method investments is not readily determinable, the evaluation of whether an investment is impaired generally consists of whether an event or change in circumstances has occurred during the reporting period that may have a significant adverse effect on the fair value of the investment (an "impairment indicator"). Impairment indicators include, but are not limited to: (1) significant deterioration in the earnings performance, credit rating, asset quality, or business prospects of the investee; (2) significant adverse change in the regulatory, economic, or technological environment of the investee; (3) significant adverse change in the general market condition of either the geographic area or the industry in which the investee operates; (4) a bona fide offer to purchase (whether solicited or unsolicited), an offer by the investee to sell, or a completed auction process for the same or similar security for an amount less than the cost of the investment; or (5) factors that raise significant concerns about the investee's ability to continue as a going concern, such as negative cash flows from operations, working capital deficiencies, or noncompliance with statutory capital requirements or debt covenants.

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