Parent Company Cash Flow Statement (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	$ 1,223.6	$ 1,031.5	$ 1,379.6
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	450.1	1,040.5	595.9
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	1,819.4	2,146.4	2,160.5
Proceeds from Sales and Maturities of Other Investments	235.0	243.4	182.2
Purchase of Fixed Maturity Securities	(2,918.4)	(3,553.6)	(3,512.8)
Purchase of Other Investments	(285.2)	(363.7)	(353.8)
Net Sales (Purchases) of Short-term Investments	(69.3)	551.3	(34.5)
Net Purchases of Property and Equipment	(114.5)	(105.5)	(105.4)
Other, Net	0.1	0.2	0.1
Net Cash Used by Investing Activities	(886.6)	(419.2)	(969.9)
Cash Flows from Financing Activities
Issuance of Long-term Debt	347.2	0	246.4
Issuance of Common Stock	12.3	11.4	4.9
Purchases of Treasury Stock	(306.0)	(317.2)	(496.7)
Dividends Paid to Stockholders	(159.4)	(146.5)	(133.8)
Other, Net	(22.9)	(27.0)	0.2
Net Cash Used by Financing Activities	(328.6)	(595.5)	(449.0)
Net Increase (Decrease) in Cash and Bank Deposits	8.4	16.8	(39.3)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	683.0	612.5	677.3
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	25.0	0	0
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	76.1	38.5	47.7
Proceeds from Sales and Maturities of Other Investments	31.9	9.4	1.0
Purchase of Fixed Maturity Securities	(118.9)	(139.8)	(99.1)
Purchase of Other Investments	(19.0)	(1.0)	(13.7)
Net Sales (Purchases) of Short-term Investments	(162.1)	269.5	40.4
Cash Distributions to Subsidiaries	(316.1)	(225.1)	(175.2)
Net Purchases of Property and Equipment	(102.5)	(78.8)	(80.4)
Other, Net	(0.2)	(6.2)	4.2
Net Cash Used by Investing Activities	(585.8)	(133.5)	(275.1)
Cash Flows from Financing Activities
Issuance of Long-term Debt	347.2	0	246.4
Issuance of Common Stock	12.3	11.4	4.9
Purchases of Treasury Stock	(306.0)	(317.2)	(496.7)
Dividends Paid to Stockholders	(159.4)	(146.5)	(133.8)
Other, Net	1.9	(0.3)	1.6
Net Cash Used by Financing Activities	(104.0)	(452.6)	(377.6)
Net Increase (Decrease) in Cash and Bank Deposits	$ (6.8)	$ 26.4	$ 24.6