Parent Company Cash Flow Statement (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	$ 1,193.7	$ 1,196.8	$ 1,237.0
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	1,181.9	1,122.8	1,427.2
Proceeds from Maturities of Fixed Maturity Securities	1,692.7	2,192.8	1,132.5
Purchase of Fixed Maturity Securities	(2,760.1)	(3,798.6)	(3,848.8)
Purchase of Other Investments	(304.1)	(332.8)	(267.7)
Net Sales (Purchases) of Short-term Investments	(254.6)	(288.1)	199.0
Other, Net	(98.0)	(110.1)	(106.6)
Net Cash Provided by (Used in) Investing Activities	(410.3)	(1,073.7)	(1,213.9)
Cash Flows from Financing Activities
Net Short-term Debt Repayments	(87.2)	0	190.5
Issuance of Long-term Debt	0	396.9	346.8
Long-term Debt Repayments	(84.4)	(78.3)	(59.8)
Issuance of Common Stock	14.8	10.0	8.0
Dividends Paid to Stockholders	(121.0)	(114.8)	(104.5)
Purchases of Treasury Stock	619.9	356.0	0
Other, Net	2.9	1.1	(1.5)
Net Cash Provided by (Used in) Financing Activities	(720.4)	(141.1)	(1.5)
Increase (Decrease) in Cash	63.0	(18.0)	21.7
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	792.9	490.1	324.1
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	133.9	3.8	49.7
Proceeds from Maturities of Fixed Maturity Securities	44.5	47.4	24.7
Purchase of Fixed Maturity Securities	0	(106.5)	(299.2)
Purchase of Other Investments	(3.0)	(22.9)	0
Net Sales (Purchases) of Short-term Investments	192.8	(266.7)	(145.1)
Cash Distributions to Subsidiaries	(166.1)	(2.4)	(1.3)
Short-term Notes Receivable from Subsidiaries	0.2	13.3	(10.5)
Acquisition of Property and Equipment	(46.2)	(70.0)	(54.7)
Other, Net	0	(18.5)	0
Net Cash Provided by (Used in) Investing Activities	156.1	(422.5)	(436.4)
Cash Flows from Financing Activities
Net Short-term Debt Repayments	(222.6)	0	(132.2)
Issuance of Long-term Debt	0	396.9	346.8
Long-term Debt Repayments	0	(10.0)	(1.8)
Issuance of Common Stock	14.8	10.0	8.0
Dividends Paid to Stockholders	(121.0)	(114.8)	(104.5)
Purchases of Treasury Stock	(619.9)	(356.0)	0
Other, Net	0	5.6	(0.7)
Net Cash Provided by (Used in) Financing Activities	(948.7)	(68.3)	115.6
Increase (Decrease) in Cash	$ 0.3	$ (0.7)	$ 3.3