Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Third Quarter
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 4,011.4	$ 2,935.3
Level 2 from Level 1	3,332.2	4,445.9
Third Quarter | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	751.9	0.0
Level 2 from Level 1	0.0	418.1
Third Quarter | 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	0.0	79.0
Third Quarter | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	236.7	316.7
Level 2 from Level 1	387.5	487.5
Third Quarter | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	3,022.8	2,618.6
Level 2 from Level 1	2,944.7	3,461.3
First, Second and Third Quarters
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,896.1	5,182.7
Level 2 from Level 1	1,317.0	986.3
First, Second and Third Quarters | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	465.8	25.0
Level 2 from Level 1	0.0	0.0
First, Second and Third Quarters | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	463.6	469.5
Level 2 from Level 1	67.1	29.3
First, Second and Third Quarters | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	4,966.7	4,688.2
Level 2 from Level 1	$ 1,249.9	$ 957.0