Investments - Unrealized Capital Losses 1 (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 277.1	$ 13.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	56.0	1.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	27.3	37.6
Total Unrealized Capital Losses	360.4	53.0
Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Amortized Cost	5,193.1	1,110.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	279.3	19.3
Six months or less below amortized cost, Number of Securities	702	141
More than six months and twelve months or less below amortized cost, Amortized Cost	503.6	49.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	46.5	2.6
More than six months and twelve months or less below amortized cost, Number of Securities	84	31
More than twelve months below amortized cost, Amortized Cost	166.7	198.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12.8	6.2
More than twelve months below amortized cost, Number of Securities	102	99
Total Amortized Cost	5,863.4	1,358.4
Total Unrealized Capital Losses	338.6	28.1
Total Number of Securities	888	271
Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Amortized Cost	66.7	15.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	15.5	3.9
Six months or less below amortized cost, Number of Securities	14	10
More than six months and twelve months or less below amortized cost, Amortized Cost	1.1	1.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.4	0.4
More than six months and twelve months or less below amortized cost, Number of Securities	2	2
More than twelve months below amortized cost, Amortized Cost	23.4	61.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.9	20.6
More than twelve months below amortized cost, Number of Securities	9	28
Total Amortized Cost	91.2	78.3
Total Unrealized Capital Losses	21.8	24.9
Total Number of Securities	25	40
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	2.4	0.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0
Total Unrealized Capital Losses	2.4	0.5
U.S. Treasuries | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	146.9	300.5
Total Unrealized Capital Losses	2.4	0.5
Total Number of Securities	3	2
U.S. Treasuries | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	0	0
Total Unrealized Capital Losses	0	0
Total Number of Securities	0	0
U.S. corporate, state and municipalities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	183.4	6.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	35.6	0.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.9	3.4
Total Unrealized Capital Losses	225.9	11.1
U.S. corporate, state and municipalities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	3,641.8	558.1
Total Unrealized Capital Losses	217.1	9.1
Total Number of Securities	478	82
U.S. corporate, state and municipalities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	35.7	4.7
Total Unrealized Capital Losses	8.8	2.0
Total Number of Securities	5	2
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	82.3	4.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	19.4	0.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.4	11.2
Total Unrealized Capital Losses	107.1	16.4
Foreign | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	1,546.0	242.7
Total Unrealized Capital Losses	98.5	5.7
Total Number of Securities	220	38
Foreign | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	39.4	36.0
Total Unrealized Capital Losses	8.6	10.7
Total Number of Securities	6	8
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	8.8	1.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.0	0.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.5	16.2
Total Unrealized Capital Losses	21.3	18.1
Residential mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	436.8	201.2
Total Unrealized Capital Losses	18.2	10.2
Total Number of Securities	157	124
Residential mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	11.6	25.2
Total Unrealized Capital Losses	3.1	7.9
Total Number of Securities	10	24
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0	0.1
Total Unrealized Capital Losses	0	0.2
Commercial mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	3.9	11.8
Total Unrealized Capital Losses	0	0.2
Total Number of Securities	3	8
Commercial mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	0	0
Total Unrealized Capital Losses	0	0
Total Number of Securities	0	0
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.2	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.5	6.7
Total Unrealized Capital Losses	3.7	6.7
Other asset-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	88.0	44.1
Total Unrealized Capital Losses	2.4	2.4
Total Number of Securities	27	17
Other asset-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	4.5	12.4
Total Unrealized Capital Losses	$ 1.3	$ 4.3
Total Number of Securities	4	6