Segment Data - Additional Information (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 143.5	$ 142.5	$ 261.2	$ 242.3
Number of reporting segments			2
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	13.5	12.8	$ 15.8	(6.2)
Insurance Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	(4.1)	(1.4)	(19.5)	(1.4)
Front Street Re (Delaware) Ltd. [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 13.1	$ 13.9	$ 9.9	$ (3.2)