Investments - Schedule of Allowance for Credit Losses (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Write-downs		$ (51.1)		$ (51.1)
Allowance for credit losses:
Balance at beginning of period	$ 43.7	5.3	$ 47.9	5.5
Provision for credit losses	(6.7)	(68.5)	(15.1)	(68.3)
Balance at end of period	47.8	$ 22.7	47.8	$ 22.7
Radioshack [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Write-downs	$ (2.6)		$ (15.2)