Parent Company Cash Flow Statement (Detail) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£)
Condensed Cash Flow Statements, Captions
Cash Provided by Operating Activities	$ 1,149.4	$ 1,116.1	$ 1,292.1
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	436.3	911.5	880.1
Proceeds from Maturities of Fixed Maturity Securities	2,560.8	2,197.8	2,417.0
Proceeds from Sales and Maturities of Other Investments	272.6	336.8	347.0
Purchase of Fixed Maturity Securities	(2,808.5)	(3,362.8)	(4,305.3)
Purchase of Other Investments	(474.3)	(576.0)	(409.5)
Net Sales (Purchases) of Short-term Investments	(356.4)	6.3	170.6
Net Purchases of Property and Equipment	(105.3)	(85.0)	(100.2)
Acquisition of Business		(140.3)	(54.3)	£ (35.9)
Net Cash Used by Investing Activities	(484.6)	(710.0)	(713.0)
Cash Flows from Financing Activities
Issuance of Long-term Debt	0.0	609.1	271.4
Long-term Debt Repayment	(63.5)	(414.0)	(226.3)
Issuance of Common Stock	10.8	8.5	6.4
Repurchase of Common Stock	(401.8)	(405.2)	(417.9)
Dividends Paid to Stockholders	(196.0)	(182.6)	(174.2)
Other, Net	(37.3)	(34.4)	(28.1)
Net Cash Used by Financing Activities	(687.8)	(418.6)	(568.7)
Net Increase (Decrease) in Cash and Bank Deposits	(23.0)	(12.5)	10.4
Parent Company
Condensed Cash Flow Statements, Captions
Cash Provided by Operating Activities	740.6	685.4	672.2
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	0.0	2.6	7.8
Proceeds from Maturities of Fixed Maturity Securities	194.3	111.2	112.2
Proceeds from Sales and Maturities of Other Investments	35.2	0.4	26.4
Purchase of Fixed Maturity Securities	(121.2)	(56.5)	(233.7)
Purchase of Other Investments	0.0	(23.3)	0.0
Net Sales (Purchases) of Short-term Investments	(99.9)	(89.8)	38.1
Cash Distributions to Subsidiaries	(79.5)	(143.6)	(231.0)
Net Purchases of Property and Equipment	(83.3)	(55.5)	(78.9)
Acquisition of Business	0.0	(140.3)	0.0
Other, Net	0.0	0.2	0.1
Net Cash Used by Investing Activities	(154.4)	(394.6)	(359.0)
Cash Flows from Financing Activities
Issuance of Long-term Debt	0.0	609.1	271.4
Long-term Debt Repayment	0.0	(350.0)	0.0
Issuance of Common Stock	10.8	8.5	6.4
Repurchase of Common Stock	(401.8)	(405.2)	(417.9)
Dividends Paid to Stockholders	(196.0)	(182.6)	(174.2)
Other, Net	0.2	(1.0)	2.9
Net Cash Used by Financing Activities	(586.8)	(321.2)	(311.4)
Net Increase (Decrease) in Cash and Bank Deposits	$ (0.6)	$ (30.4)	$ 1.8