Securities - Securities by Maturity (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Marketable Securities Fixed Maturities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 830
Amortized Cost of Securities Available for sale, Due after one through five years	12,112
Amortized Cost of Securities Available for sale, Due after five through ten years	221
Amortized Cost of Securities Available for sale, Due after ten years	3
Total Amortized Cost of Securities Available for sale	13,166
Fair Value of Securities Available for sale, Due in one year or less	844
Fair Value of Securities Available for sale, Due after one through five years	12,428
Fair Value of Securities Available for sale, Due after five through ten years	221
Fair Value of Securities Available for sale, Due after ten years	3
Total Fair Value of Securities Available for sale	13,496
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	10
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	3,711
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years
Amortized Cost of Held-to-Maturity Securities, Due after ten years
Total Amortized Cost of Held-to-Maturity securities	3,721
Fair Value of Held-to-Maturity Securities, Due in one year or less	10
Fair Value of Held-to-Maturity Securities, Due after one through five years	3,769
Fair Value of Held-to-Maturity Securities, Due after five through ten years
Fair Value of Held-to-Maturity Securities, Due after ten years
Total Fair Value of Held-to-Maturity Securities	$ 3,779

Amount of available-for-sale debt securities at cost, net of adjustments, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 20

-Subparagraph c

-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.