Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	$ 5,290.1	$ 3,006.8
Level 2 from Level 1 Transfers	1,238.3	967.1
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	219.6	163.2
Level 2 from Level 1 Transfers	0	121.5
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	606.7	287.7
Level 2 from Level 1 Transfers	48.9	54.4
Corporate Debt Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	4,463.8	2,555.9
Level 2 from Level 1 Transfers	$ 1,189.4	$ 791.2