Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,712.7	[1]	$ 1,612.3
Percent of Total	100.00%		100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	34.0		28.0
Percent of Total	2.00%		1.80%
Industrial
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	513.4		502.0
Percent of Total	30.00%		31.10%
Mixed
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	78.4		93.5
Percent of Total	4.60%		5.80%
Office
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	713.4		659.3
Percent of Total	41.60%		40.90%
Retail
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	373.5		322.4
Percent of Total	21.80%		20.00%
Other
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 0		$ 7.1
Percent of Total	0.00%		0.40%

[1] Amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and accretion of discounts.