Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Available for Sale Abstract [Abstract]
Level 1 from Level 2	$ 2,641.0	$ 3,148.4	$ 3,239.8	$ 3,731.6
Level 2 from Level 1	3,078.1	2,527.5	2,523.5	1,698.4
United States Government and Government Agencies and Authorities
Available for Sale Abstract [Abstract]
Level 1 from Level 2	177.2	17.1	169.8	110.8
Level 2 from Level 1	16.1	20.5	0	0
States, Municipalities, and Political Subdivisions
Available for Sale Abstract [Abstract]
Level 1 from Level 2	0	123.7	25.4	55.4
Level 2 from Level 1	20.0	40.0	301.9	59.5
Foreign Governments
Available for Sale Abstract [Abstract]
Level 1 from Level 2	0		0
Level 2 from Level 1	0		0.7
Public Utilities
Available for Sale Abstract [Abstract]
Level 1 from Level 2	665.7	678.9	710.5	616.1
Level 2 from Level 1	694.8	579.4	488.6	581.0
All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Level 1 from Level 2	1,798.1	2,328.7	2,334.1	2,949.3
Level 2 from Level 1	2,347.2	1,887.6	1,732.3	1,052.4
Redeemable Preferred Stocks [Member]
Available for Sale Abstract [Abstract]
Level 1 from Level 2		0		0
Level 2 from Level 1		$ 0		$ 5.5