Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	$ 38,803.4	$ 38,289.6
Fair value of fixed maturity securities	45,064.9	42,344.4
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	983.5	1,028.6
Accumulated Gross Unrealized Gain on Securities	255.5	173.1
Accumulated Gross Unrealized Loss on Securities	0.5	5.6
Fair value of fixed maturity securities	1,238.5	1,196.1
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	1,745.0	1,706.0
Accumulated Gross Unrealized Gain on Securities	377.6	117.2
Accumulated Gross Unrealized Loss on Securities	1.1	40.0
Fair value of fixed maturity securities	2,121.5	1,783.2
Foreign Governments
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	1,101.1	1,226.4
Accumulated Gross Unrealized Gain on Securities	206.3	149.6
Accumulated Gross Unrealized Loss on Securities	0	2.8
Fair value of fixed maturity securities	1,307.4	1,373.2
Public Utilities
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	7,046.1	7,121.7
Accumulated Gross Unrealized Gain on Securities	1,505.4	901.2
Accumulated Gross Unrealized Loss on Securities	0.9	25.7
Fair value of fixed maturity securities	8,550.6	7,997.2
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	2,224.9	1,858.7
Accumulated Gross Unrealized Gain on Securities	207.0	184.6
Accumulated Gross Unrealized Loss on Securities	0.1	4.0
Fair value of fixed maturity securities	2,431.8	2,039.3
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	25,658.8	25,315.2
Accumulated Gross Unrealized Gain on Securities	3,828.6	2,828.3
Accumulated Gross Unrealized Loss on Securities	122.2	225.8
Fair value of fixed maturity securities	29,365.2	27,917.7
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	44.0	33.0
Accumulated Gross Unrealized Gain on Securities	5.9	4.7
Accumulated Gross Unrealized Loss on Securities	0	0
Fair value of fixed maturity securities	49.9	37.7
Fixed Maturity Securities
Schedule of Available-for-sale Securities
Fixed Maturity Securities, Amortized Cost	38,803.4	38,289.6
Accumulated Gross Unrealized Gain on Securities	6,386.3	4,358.7
Accumulated Gross Unrealized Loss on Securities	124.8	303.9
Fair value of fixed maturity securities	$ 45,064.9	$ 42,344.4

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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