Segment Data - Additional Information (Detail) $ in Millions	3 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 117.7	$ 99.8
Number of reporting segments	2
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 2.3	(19.0)
Insurance Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	(15.4)	0.0
Front Street Re (Delaware) Ltd. [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ (3.2)	$ (17.1)