Investments - Additional Information (Detail) In Millions, unless otherwise specified	12 Months Ended						12 Months Ended			12 Months Ended
	Dec. 31, 2012 USD ($) sqft	Dec. 31, 2011 USD ($)	Jul. 14, 2011 GBP (£)	Dec. 31, 2010 USD ($)	Dec. 31, 2012 Overseas USD ($)	Dec. 31, 2011 Overseas USD ($)	Dec. 31, 2012 Capital Loss Carryforward USD ($)	Dec. 31, 2012 Federal Home Loan Mortgage Corp. USD ($)	Dec. 31, 2011 Federal Home Loan Mortgage Corp. USD ($)	Dec. 31, 2012 Security Lending Program USD ($)	Dec. 31, 2011 Security Lending Program USD ($)	Dec. 31, 2012 United States Government and agencies and authorities USD ($)	Dec. 31, 2011 United States Government and agencies and authorities USD ($)	Dec. 31, 2012 Collateralized Borrowings And Other Arrangements USD ($)	Dec. 31, 2011 Collateralized Borrowings And Other Arrangements USD ($)	Dec. 31, 2012 Federal Home Loan Bank Borrowings USD ($)	Dec. 31, 2011 Federal Home Loan Bank Borrowings USD ($)
Schedule of Investments [Line Items]
Net unrealized gains on impaired securities	$ 20.5	$ 25.1
Securities on loan, fair value										29.2	24.1	115.7	103.5	213.8	212.9	200.8	205.7
Fair value of the loaned securities, description										The fair value of the loaned securities is monitored on a daily basis, and the collateral is maintained at a level of at least 102% of the fair value of the loaned securities.
Fixed maturities deposit, amortized cost												106.2	97.0
Restricted assets, fixed maturities	497	372
Restricted assets, cash and cash equivalents	3	94
Loss on derivative		11.3					5.1
Derivative instruments, gain recognized							0.7
Notional amount of foreign currency contract			297.9
Net appreciation on other invested assets	2.1	1.9		1.8
Cash flow hedge loss reclassified into expense		0.2
Derivative instruments, gain recognized in income		6.1
Net depreciation on fixed maturity derivative instruments	1.0	1.2
Equity investment, fair value								415.7	455.0
Concentration of Investment in a Single Investee, Maximum	10.00%
Amount of land purchased for development, in acres				11
Land purchased for development, value				5
Office building, square foot	200,000
Sale of office building and adjacent garage	75.9
Gain recognized on sale of office building and adjacent garage	3.3
FHLB loan																46.3
Contractual investment commitments	17.3
Overseas deposits	$ 210.3	$ 190.2			$ 169.2	$ 135.1