Parent Company Cash Flow Statement (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions
Cash Provided by Operating Activities	$ 1,292.1	$ 1,223.6	$ 1,031.5
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	880.1	450.1	1,040.5
Proceeds from Maturities of Fixed Maturity Securities	2,417.0	1,819.4	2,146.4
Proceeds from Sales and Maturities of Other Investments	347.0	235.0	243.4
Purchase of Fixed Maturity Securities	(4,305.3)	(2,918.4)	(3,553.6)
Purchase of Other Investments	(409.5)	(285.2)	(363.7)
Net Sales (Purchases) of Short-term Investments	170.6	(69.3)	551.3
Net Purchases of Property and Equipment	(100.2)	(114.5)	(105.5)
Other, Net	0.0	0.1	0.2
Net Cash Used by Investing Activities	(713.0)	(886.6)	(419.2)
Cash Flows from Financing Activities
Issuance of Long-term Debt	271.4	347.2	0.0
Issuance of Common Stock	6.4	12.3	11.4
Repurchase of Common Stock	(417.9)	(306.0)	(317.2)
Dividends Paid to Stockholders	(174.2)	(159.4)	(146.5)
Other, Net	(28.1)	(22.9)	(27.0)
Cash Used by Financing Activities	(568.7)	(328.6)	(595.5)
Net Increase in Cash and Bank Deposits	10.4	8.4	16.8
Parent Company
Condensed Financial Statements, Captions
Cash Provided by Operating Activities	672.2	683.0	612.5
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	7.8	25.0	0.0
Proceeds from Maturities of Fixed Maturity Securities	112.2	76.1	38.5
Proceeds from Sales and Maturities of Other Investments	26.4	31.9	9.4
Purchase of Fixed Maturity Securities	(233.7)	(118.9)	(139.8)
Purchase of Other Investments	0.0	(19.0)	(1.0)
Net Sales (Purchases) of Short-term Investments	38.1	(162.1)	269.5
Cash Distributions to Subsidiaries	(231.0)	(316.1)	(225.1)
Net Purchases of Property and Equipment	(78.9)	(102.5)	(78.8)
Other, Net	0.1	(0.2)	(6.2)
Net Cash Used by Investing Activities	(359.0)	(585.8)	(133.5)
Cash Flows from Financing Activities
Issuance of Long-term Debt	271.4	347.2	0.0
Issuance of Common Stock	6.4	12.3	11.4
Repurchase of Common Stock	(417.9)	(306.0)	(317.2)
Dividends Paid to Stockholders	(174.2)	(159.4)	(146.5)
Other, Net	2.9	1.9	(0.3)
Cash Used by Financing Activities	(311.4)	(104.0)	(452.6)
Net Increase in Cash and Bank Deposits	$ 1.8	$ (6.8)	$ 26.4