Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
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	$ 1,082.2	$ 1,073.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	6,228.2	6,267.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	13,336.4	12,573.4
Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost	18,696.2	18,837.2
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal	39,343.0	38,751.4
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	1,461.8	1,523.8
Amortized Cost of Fixed Maturity Securities	40,804.8	40,275.2
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain	17.2	14.5
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	433.8	300.6
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	1,477.5	795.0
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	3,750.2	2,315.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	5,678.7	3,425.7
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	105.1	67.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,783.8	3,492.9
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss	3.3	8.4
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	52.1	80.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	28.8	303.9
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	63.7	355.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	147.9	748.1
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	0.5	8.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	148.4	756.4
Fair Value Maturity Distribution
Fixed Maturity Securities	46,440.2	43,011.7
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	1,065.7	1,020.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	6,232.6	5,186.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	13,952.1	6,812.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	21,305.6	15,779.7
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	42,556.0	28,799.4
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	1,495.8	1,299.6
Fixed Maturity Securities	44,051.8	30,099.0
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	30.4	59.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	377.3	1,301.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	833.0	6,251.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	1,077.1	5,017.5
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	2,317.8	12,629.6
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	70.6	283.1
Fixed Maturity Securities	$ 2,388.4	$ 12,912.7

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