Parent Company Cash Flow Statement (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	$ 1,379.6	$ 1,193.7	$ 1,196.8
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	595.9	1,181.9	1,122.8
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	2,160.5	1,692.7	2,192.8
Proceeds from Sales and Maturities of Other Investments	182.2	131.9	140.3
Purchase of Fixed Maturity Securities	(3,512.8)	(2,760.1)	(3,798.6)
Purchase of Other Investments	(353.8)	(304.1)	(332.8)
Net Purchases of Short-term Investments	(80.1)	(254.6)	(288.1)
Other, Net	105.3	98.0	110.1
Net Cash Provided by (Used in) Investing Activities	(1,113.4)	(410.3)	(1,073.7)
Cash Flows from Financing Activities
Proceeds from (Repayments of) Short-term Debt	143.5	87.2	0
Issuance of Long-term Debt	246.4	0	396.9
Long-term Debt Repayments	(70.0)	(84.4)	(78.3)
Issuance of Common Stock	4.9	14.8	10.0
Purchases of Treasury Stock	(496.7)	(619.9)	(356.0)
Dividends Paid to Stockholders	(133.8)	(121.0)	(114.8)
Other, Net	0.2	2.9	1.1
Net Cash Used by Financing Activities	(305.5)	(720.4)	(141.1)
Net Increase (Decrease) in Cash and Bank Deposits	(39.3)	63.0	(18.0)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	677.3	827.8	490.1
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	0	133.9	3.8
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	47.7	44.5	47.4
Proceeds from Sales and Maturities of Other Investments	1.0	0
Purchase of Fixed Maturity Securities	(99.1)	0	(106.5)
Purchase of Other Investments	(13.7)	(3.0)	(22.9)
Net Purchases of Short-term Investments	40.4	192.8	(266.7)
Cash Distributions to Subsidiaries	(175.2)	(166.1)	(2.4)
Short-term Notes Receivable from Subsidiaries	0	0.2	13.3
Acquisition of Property and Equipment	(80.4)	(81.1)	(70.0)
Other, Net	(0.1)	0	18.5
Net Cash Provided by (Used in) Investing Activities	(279.2)	121.2	(422.5)
Cash Flows from Financing Activities
Proceeds from (Repayments of) Short-term Debt	4.1	(222.6)	0
Issuance of Long-term Debt	246.4	0	396.9
Long-term Debt Repayments	0	0	(10.0)
Issuance of Common Stock	4.9	14.8	10.0
Purchases of Treasury Stock	(496.7)	(619.9)	(356.0)
Dividends Paid to Stockholders	(133.8)	(121.0)	(114.8)
Other, Net	1.6	0	5.6
Net Cash Used by Financing Activities	(373.5)	(948.7)	(68.3)
Net Increase (Decrease) in Cash and Bank Deposits	$ 24.6	$ 0.3	$ (0.7)