Investments - Schedule of Allowance for Credit Losses (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2015
Allowance for credit losses:
Balance at beginning of period	$ 47.9	$ 5.5	$ 5.5
Provision for credit losses	(7.9)	0.2
Balance at end of period	$ 43.7	$ 5.3	$ 47.9