Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
First and Second Quarters
Fixed Maturity Securities
Level 1 from Level 2	$ 3,162.3	$ 3,570.8
Level 2 from Level 1	1,098.8	2,284.9
Second Quarter
Fixed Maturity Securities
Level 1 from Level 2	1,795.7	3,026.8
Level 2 from Level 1	3,665.7	2,534.4
United States Government and Government Agencies and Authorities | First and Second Quarters
Fixed Maturity Securities
Level 1 from Level 2	48.0	205.9
Level 2 from Level 1	0	20.5
United States Government and Government Agencies and Authorities | Second Quarter
Fixed Maturity Securities
Level 1 from Level 2	21.7	426.8
Level 2 from Level 1	164.6	60.5
States, Municipalities, and Political Subdivisions | First and Second Quarters
Fixed Maturity Securities
Level 1 from Level 2	0	41.9
Level 2 from Level 1	53.1	74.8
States, Municipalities, and Political Subdivisions | Second Quarter
Fixed Maturity Securities
Level 1 from Level 2	0	43.0
Level 2 from Level 1	31.8	62.9
Foreign Government Debt Securities [Member] | First and Second Quarters
Fixed Maturity Securities
Level 1 from Level 2	0	20.7
Level 2 from Level 1	0	0
Foreign Government Debt Securities [Member] | Second Quarter
Fixed Maturity Securities
Level 1 from Level 2	0	21.7
Level 2 from Level 1	0	0
Public Utilities | First and Second Quarters
Fixed Maturity Securities
Level 1 from Level 2	577.9	479.0
Level 2 from Level 1	53.5	483.6
Public Utilities | Second Quarter
Fixed Maturity Securities
Level 1 from Level 2	367.1	382.4
Level 2 from Level 1	584.7	743.6
All Other Corporate Bonds | First and Second Quarters
Fixed Maturity Securities
Level 1 from Level 2	2,536.4	2,823.3
Level 2 from Level 1	992.2	1,706.0
All Other Corporate Bonds | Second Quarter
Fixed Maturity Securities
Level 1 from Level 2	1,406.9	2,152.9
Level 2 from Level 1	2,884.6	1,667.4
Equity Securites | Second Quarter
Fixed Maturity Securities
Level 1 from Level 2	0	0
Level 2 from Level 1	$ 0	$ 7.4