Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Third Quarter
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	$ 2,399.8	$ 3,665.7
Level 1 from Level 2 Transfers	3,826.1	1,795.7
Third Quarter | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	0	164.6
Level 1 from Level 2 Transfers	169.6	21.7
Third Quarter | States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	0	31.8
Level 1 from Level 2 Transfers	0	0
Third Quarter | Public Utilities
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	218.8	375.7
Level 1 from Level 2 Transfers	415.1	239.3
Third Quarter | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	2,181.0	3,093.6
Level 1 from Level 2 Transfers	3,241.4	1,534.7
First, Second, and Third Quarters
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	636.4	1,098.8
Level 1 from Level 2 Transfers	4,736.0	3,162.3
First, Second, and Third Quarters | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	0	0
Level 1 from Level 2 Transfers	163.2	48.0
First, Second, and Third Quarters | States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	0	53.1
Level 1 from Level 2 Transfers	0	0
First, Second, and Third Quarters | Public Utilities
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	88.1	33.5
Level 1 from Level 2 Transfers	470.8	405.5
First, Second, and Third Quarters | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 2 from Level 1 Transfers	548.3	1,012.2
Level 1 from Level 2 Transfers	$ 4,102.0	$ 2,708.8