Legal Matters - Additional Information (Detail) (USD $)	6 Months Ended	0 Months Ended	6 Months Ended		12 Months Ended	0 Months Ended
	Mar. 31, 2014	Jan. 27, 2014 Class plaintiffs	Mar. 31, 2014 Class plaintiffs	Mar. 31, 2013 Class plaintiffs	Sep. 30, 2013 Class plaintiffs	Dec. 16, 2013 Interchage Litigation
Loss Contingencies [Line Items]
Payments on litigation matters			$ 0	$ 4,033,000,000	$ 4,033,000,000
Adjustment to payments for litigation		1,056,000,000
Damages sought by plaintiff from all defendant parties						54,000,000,000
Estimated compensatory damages	$ 1,000,000,000