Parent Company Cash Flow Statement (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	$ 1,031.5	$ 1,379.6	$ 1,193.7
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	1,040.5	595.9	1,181.9
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	2,146.4	2,160.5	1,692.7
Proceeds from Sales and Maturities of Other Investments	243.4	182.2	131.9
Purchase of Fixed Maturity Securities	(3,553.6)	(3,512.8)	(2,760.1)
Purchase of Other Investments	(363.7)	(353.8)	(304.1)
Net Sales (Purchases) of Short-term Investments	552.4	(80.1)	(254.6)
Net Purchases of Property and Equipment	(105.5)	(105.4)	(98.2)
Other, Net	0.2	0.1	0.2
Net Cash Provided by (Used in) Investing Activities	(39.9)	(1,113.4)	(410.3)
Cash Flows from Financing Activities
Proceeds from (Repayments of) Short-term Debt	(379.3)	143.5	87.2
Issuance of Long-term Debt	0	246.4	0
Issuance of Common Stock	11.4	4.9	14.8
Purchases of Treasury Stock	(317.2)	(496.7)	(619.9)
Dividends Paid to Stockholders	(146.5)	(133.8)	(121.0)
Other, Net	(27.0)	0.2	2.9
Net Cash Used by Financing Activities	(974.8)	(305.5)	(720.4)
Net Increase (Decrease) in Cash and Bank Deposits	16.8	(39.3)	63.0
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	612.5	677.3	827.8
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	0	0	133.9
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	38.5	47.7	44.5
Proceeds from Sales and Maturities of Other Investments	9.4	1.0	0
Purchase of Fixed Maturity Securities	(139.8)	(99.1)	0
Purchase of Other Investments	0	(13.7)	(3.0)
Net Sales (Purchases) of Short-term Investments	269.5	40.4	192.8
Cash Distributions to Subsidiaries	(225.1)	(175.2)	(166.1)
Net Purchases of Property and Equipment	(78.8)	(80.4)	(81.1)
Other, Net	(0.9)	0.1	0.2
Net Cash Provided by (Used in) Investing Activities	(127.2)	(279.2)	121.2
Cash Flows from Financing Activities
Proceeds from (Repayments of) Short-term Debt	(6.3)	4.1	(222.6)
Issuance of Long-term Debt	0	246.4	0
Issuance of Common Stock	11.4	4.9	14.8
Purchases of Treasury Stock	(317.2)	(496.7)	(619.9)
Dividends Paid to Stockholders	(146.5)	(133.8)	(121.0)
Other, Net	(0.3)	1.6	0
Net Cash Used by Financing Activities	(458.9)	(373.5)	(948.7)
Net Increase (Decrease) in Cash and Bank Deposits	$ 26.4	$ 24.6	$ 0.3