Investments (Schedule of Available-for-sale Securities Reconciliation) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	$ 6,785.1	$ 6,815.2
Gross Unrealized Gains	309.8	246.9
Gross Unrealized Loss	36.4	91.5
Fair Value	7,058.5	6,970.6
OTTI Unrealized Losses	7.8	10.2
US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	403.5	417.5
Gross Unrealized Gains	6.0	3.3
Gross Unrealized Loss	6.3	14.2
Fair Value	403.2	406.6
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	319.7	304.5
Gross Unrealized Gains	2.6	2.1
Gross Unrealized Loss	1.3	1.6
Fair Value	321.0	305.0
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	1,080.4	1,108.0
Gross Unrealized Gains	59.5	37.4
Gross Unrealized Loss	6.4	19.1
Fair Value	1,133.5	1,126.3
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	3,718.3	3,690.2
Gross Unrealized Gains	205.4	171.5
Gross Unrealized Loss	15.0	37.5
Fair Value	3,908.7	3,824.2
OTTI Unrealized Losses	7.4	8.6
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	697.3	722.8
Gross Unrealized Gains	22.1	20.1
Gross Unrealized Loss	5.8	14.1
Fair Value	713.6	728.8
OTTI Unrealized Losses	0.4	1.6
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	395.1	405.9
Gross Unrealized Gains	12.5	10.5
Gross Unrealized Loss	1.5	4.8
Fair Value	406.1	411.6
Asset-backed Securities, Securitized Loans and Receivables [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	170.8	166.3
Gross Unrealized Gains	1.7	2.0
Gross Unrealized Loss	0.1	0.2
Fair Value	172.4	168.1
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	446.4	366.5
Gross Unrealized Gains	84.9	66.9
Gross Unrealized Loss	0.8	3.2
Fair Value	$ 530.5	$ 430.2