Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
First, Second, and Third Quarters [Member] [Member]
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 5,182.7	$ 4,191.1
Level 2 from Level 1	986.3	1,213.9
First, Second, and Third Quarters [Member] [Member] | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	25.0	219.7
Level 2 from Level 1	0.0	0.0
First, Second, and Third Quarters [Member] [Member] | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	469.5	306.1
Level 2 from Level 1	29.3	57.0
First, Second, and Third Quarters [Member] [Member] | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	4,688.2	3,665.3
Level 2 from Level 1	957.0	1,156.9
Third Quarter [Member]
Transfers Between Level 1 and Level 2
Level 1 from Level 2	2,935.3	2,285.4
Level 2 from Level 1	4,445.9	3,165.4
Third Quarter [Member] | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	0.0
Level 2 from Level 1	418.1	0.0
Third Quarter [Member] | States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	0.0
Level 2 from Level 1	79.0	0.0
Third Quarter [Member] | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	316.7	157.2
Level 2 from Level 1	487.5	231.8
Third Quarter [Member] | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	2,618.6	2,128.2
Level 2 from Level 1	$ 3,461.3	$ 2,933.6