Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Third Quarter
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 2,953.4	$ 4,011.4
Level 2 from Level 1	2,880.6	3,332.2
Third Quarter | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	225.5	751.9
Level 2 from Level 1	38.1	0.0
Third Quarter | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	77.7	236.7
Level 2 from Level 1	323.4	387.5
Third Quarter | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	2,650.2	3,022.8
Level 2 from Level 1	2,519.1	2,944.7
First, Second, and Third Quarters
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,187.7	5,896.1
Level 2 from Level 1	1,257.5	1,317.0
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	233.4	465.8
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	244.0	463.6
Level 2 from Level 1	57.4	67.1
First, Second, and Third Quarters | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	4,710.3	4,966.7
Level 2 from Level 1	1,200.1	1,249.9
First, Second, and Third Quarters | Equity Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	0.0
Level 2 from Level 1	$ 0.2	$ 0.0