Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	$ 1,039.7	$ 1,073.3
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	6,433.0	6,267.5
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	12,907.5	12,573.4
Fixed Maturity Securities, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost	18,620.4	18,837.2
Fixed Maturity Securities, Maturities Amortized Cost Gross Subtotal	39,000.6	38,751.4
Fixed Maturity Securities, Maturity, without Single Maturity Date, Amortized Cost	1,473.6	1,523.8
Fixed Maturity Securities, Amortized Cost	40,474.2	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	15.1	14.5
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	387.7	300.6
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	1,095.3	795.0
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	2,997.6	2,315.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	4,495.7	3,425.7
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	84.5	67.2
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	4,580.2	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	9.1	8.4
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	49.6	80.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	80.8	303.9
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	130.3	355.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	269.8	748.1
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	2.0	8.3
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	271.8	756.4
Fair Value Maturity Distribution
Fixed Maturity Securities	44,782.6	43,011.7
Available for Sale Securities Unrealized Gain Position Fair Value
Fair Value Maturity Distribution
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	972.8	1,020.1
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	6,244.3	5,186.9
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	11,243.1	6,812.7
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	18,888.6	15,779.7
Fixed Maturity Securities, Fair Value Gross Subtotal	37,348.8	28,799.4
Fixed Maturity Securities, Fair Value	1,370.7	1,299.6
Fixed Maturity Securities	38,719.5	30,099.0
Available for Sale Securities Unrealized Loss Position Fair Value
Fair Value Maturity Distribution
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	72.9	59.3
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	526.8	1,301.0
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	2,678.9	6,251.8
Fixed Maturity Securities, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	2,599.1	5,017.5
Fixed Maturity Securities, Fair Value Gross Subtotal	5,877.7	12,629.6
Fixed Maturity Securities, Fair Value	185.4	283.1
Fixed Maturity Securities	$ 6,063.1	$ 12,912.7

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