Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,612.3	[1]	$ 1,516.8
Percent of Total	100.00%		100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	28.0		33.7
Percent of Total	1.80%		2.20%
Industrial
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	502.0		458.2
Percent of Total	31.10%		30.20%
Mixed
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	93.5		95.8
Percent of Total	5.80%		6.30%
Office
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	659.3		634.7
Percent of Total	40.90%		41.90%
Retail
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	322.4		286.9
Percent of Total	20.00%		18.90%
Other
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 7.1		$ 7.5
Percent of Total	0.40%		0.50%

[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.