Mortgage Loans by Geographic Region (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%
New England
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	114.3		147.0
Percent of Total	6.70%		9.10%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	160.0		174.1
Percent of Total	9.30%		10.80%
East North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	224.7		212.7
Percent of Total	13.10%		13.20%
West North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	160.8		151.2
Percent of Total	9.40%		9.40%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	440.9		383.8
Percent of Total	25.70%		23.80%
East South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	79.6		52.4
Percent of Total	4.70%		3.30%
West South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	159.5		160.4
Percent of Total	9.30%		9.90%
Mountain
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	90.5		69.5
Percent of Total	5.30%		4.30%
Pacific
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 282.4		$ 261.2
Percent of Total	16.50%		16.20%

[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.