Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	$ 992.6	$ 1,625.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	6,062.7	5,579.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	12,558.1	12,091.1
Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost	18,708.8	18,611.2
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal	38,322.2	37,907.3
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	1,591.0	1,873.2
Debt Securities, Available-for-sale, Amortized Cost	39,913.2	39,780.5
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain	13.5	30.9
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	298.6	453.6
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	826.6	1,169.8
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	2,458.9	4,041.4
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	3,597.6	5,695.7
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	57.1	105.1
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	3,654.7	5,800.8
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss	5.4	3.1
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	41.6	18.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	185.4	53.2
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	243.1	44.0
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	475.5	118.8
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	21.1	4.7
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	496.6	123.5
Fair Value Maturity Distribution
Fixed Maturity Securities	43,071.3	45,457.8
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Fair Value Maturity Distribution
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	949.8	1,638.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	5,345.8	5,810.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	7,993.7	11,916.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	16,266.6	21,333.1
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	30,555.9	40,699.3
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	898.7	1,576.4
Fixed Maturity Securities	31,454.6	42,275.7
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	50.9	14.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	973.9	204.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	5,205.6	1,291.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	4,658.0	1,275.5
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	10,888.4	2,784.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	728.3	397.2
Fixed Maturity Securities	$ 11,616.7	$ 3,182.1

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