Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Available for Sale Abstract [Abstract]
Level 1 from Level 2	$ 2,473.4	$ 2,495.7	$ 3,348.3	$ 3,381.2
Level 2 from Level 1	3,109.5	2,700.9	2,357.7	1,995.0
United States Government and Government Agencies and Authorities
Available for Sale Abstract [Abstract]
Level 1 from Level 2	0	18.3	16.1	114.3
Level 2 from Level 1	0	122.9	0	0
States, Municipalities, and Political Subdivisions
Available for Sale Abstract [Abstract]
Level 1 from Level 2	0	0	25.4	0
Level 2 from Level 1	27.0	26.0	301.9	75.6
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	531.8	485.9	698.3	577.1
Level 2 from Level 1	762.1	697.1	526.9	655.8
All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Level 1 from Level 2	1,941.6	1,991.5	2,608.5	2,689.8
Level 2 from Level 1	2,320.4	1,849.1	1,528.2	1,258.1
Redeemable Preferred Stocks [Member]
Available for Sale Abstract [Abstract]
Level 1 from Level 2		0		0
Level 2 from Level 1		5.8		5.5
Equity Securities [Member]
Available for Sale Abstract [Abstract]
Level 1 from Level 2	9.9		9.0
Level 2 from Level 1	$ 0		$ 0