Amount of earned premiums assumed from other entities.

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Schedule IV Reinsurance (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment
Direct Premiums, Life Insurance in Force	$ 790,952.1us-gaap_LifeInsuranceInForceGross				$ 781,495.9us-gaap_LifeInsuranceInForceGross				$ 790,952.1us-gaap_LifeInsuranceInForceGross	$ 781,495.9us-gaap_LifeInsuranceInForceGross	$ 832,587.5us-gaap_LifeInsuranceInForceGross
Ceded Premiums, Life Insurance in Force	24,416.7us-gaap_LifeInsuranceInForceCeded				25,904.7us-gaap_LifeInsuranceInForceCeded				24,416.7us-gaap_LifeInsuranceInForceCeded	25,904.7us-gaap_LifeInsuranceInForceCeded	28,658.7us-gaap_LifeInsuranceInForceCeded
Assumed Premiums, Life Insurance in Force	999.7us-gaap_LifeInsuranceInForceAssumed				1,026.2us-gaap_LifeInsuranceInForceAssumed				999.7us-gaap_LifeInsuranceInForceAssumed	1,026.2us-gaap_LifeInsuranceInForceAssumed	1,073.8us-gaap_LifeInsuranceInForceAssumed
Premiums, Net, Life Insurance in Force	767,535.1us-gaap_LifeInsuranceInForceNet				756,617.4us-gaap_LifeInsuranceInForceNet				767,535.1us-gaap_LifeInsuranceInForceNet	756,617.4us-gaap_LifeInsuranceInForceNet	805,002.6us-gaap_LifeInsuranceInForceNet
Life Insurance in Force Premiums, Percentage Assumed to Net	0.10%				0.10%				0.10%	0.10%	0.10%
Gross Amount									7,899.3us-gaap_DirectPremiumsEarned	7,777.3us-gaap_DirectPremiumsEarned	7,736.0us-gaap_DirectPremiumsEarned
Reinsurance Ceded									291.9us-gaap_CededPremiumsEarned	355.8us-gaap_CededPremiumsEarned	230.8us-gaap_CededPremiumsEarned
Assumed Premiums Earned									189.8us-gaap_AssumedPremiumsEarned	203.2us-gaap_AssumedPremiumsEarned	210.9us-gaap_AssumedPremiumsEarned
Net Amount	1,967.9us-gaap_PremiumsEarnedNet	1,947.2us-gaap_PremiumsEarnedNet	1,943.6us-gaap_PremiumsEarnedNet	1,938.5us-gaap_PremiumsEarnedNet	1,890.7us-gaap_PremiumsEarnedNet	1,897.3us-gaap_PremiumsEarnedNet	1,905.8us-gaap_PremiumsEarnedNet	1,930.9us-gaap_PremiumsEarnedNet	7,797.2us-gaap_PremiumsEarnedNet	7,624.7us-gaap_PremiumsEarnedNet	7,716.1us-gaap_PremiumsEarnedNet
Percentage of Amount Assumed to Net									2.40%us-gaap_PremiumsPercentageAssumedToNet	2.70%us-gaap_PremiumsPercentageAssumedToNet	2.70%us-gaap_PremiumsPercentageAssumedToNet
Life Insurance Product Line
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									2,056.4	2,018.7	1,979.1
Reinsurance Ceded									194.8	253.6	141.4
Assumed Premiums Earned									9.6	10.0	10.3
Net Amount									1,871.2	1,775.1	1,848.0
Percentage of Amount Assumed to Net									0.50%	0.60%	0.60%
Accident, Sickness, and Disability Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									5,842.9	5,758.6	5,756.9
Reinsurance Ceded									97.1	102.2	89.4
Assumed Premiums Earned									180.2	193.2	200.6
Net Amount									$ 5,926.0	$ 5,849.6	$ 5,868.1
Percentage of Amount Assumed to Net									3.00%	3.30%	3.40%