Mortgage Loans by Geographic Region (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%
New England
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	147.0		146.8
Percent of Total	9.10%		9.70%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	174.1		184.8
Percent of Total	10.80%		12.20%
East North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	212.7		171.7
Percent of Total	13.20%		11.30%
West North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	151.2		134.6
Percent of Total	9.40%		8.90%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	383.8		372.0
Percent of Total	23.80%		24.50%
East South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	52.4		26.9
Percent of Total	3.30%		1.80%
West South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	160.4		171.8
Percent of Total	9.90%		11.30%
Mountain
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	69.5		60.7
Percent of Total	4.30%		4.00%
Pacific
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 261.2		$ 247.5
Percent of Total	16.20%		16.30%

[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.