Investments in Marketable Securities - Schedule of Realized Gains and Losses on Sales from AFS Presented in Our Consolidated Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended
Jun. 30, 2017	Jun. 30, 2016
Available-for-sale securities:
Realized gains on sales	$ 42	$ 13
Realized losses on sales	$ (1)	$ 0

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration