Investments - Unrealized Capital Losses 1 (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 13.7	$ 45.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.7	16.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	37.6	91.3
Total Unrealized Capital Losses	53.0	153.4
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	1,110.8	1,197.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	19.3	46.9
Six months or less below amortized cost, Number of Securities	141	256
More than six months and twelve months or less below amortized cost, Amortized Cost	49.5	270.3
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.6	13.9
More than six months and twelve months or less below amortized cost, Number of Securities	31	52
More than twelve months below amortized cost, Amortized Cost	198.1	355.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.2	26.7
More than twelve months below amortized cost, Number of Securities	99	129
Total Amortized Cost	1,358.4	1,823.1
Total Unrealized Capital Losses	28.1	87.5
More than twelve months below amortized cost, Number of Securities	271	437
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	15.2	60.1
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	3.9	16.9
Six months or less below amortized cost, Number of Securities	10	31
More than six months and twelve months or less below amortized cost, Amortized Cost	1.5	25.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.4	9.1
More than six months and twelve months or less below amortized cost, Number of Securities	2	9
More than twelve months below amortized cost, Amortized Cost	61.6	103.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	20.6	39.9
More than twelve months below amortized cost, Number of Securities	28	37
Total Amortized Cost	78.3	189.1
Total Unrealized Capital Losses	24.9	65.9
More than twelve months below amortized cost, Number of Securities	40	77
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5	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	0	0
Total Unrealized Capital Losses	0.5	0
U.S. Treasuries | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	300.5	0
Total Unrealized Capital Losses	0.5	0
More than twelve months below amortized cost, Number of Securities	2	0
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
More than twelve months below amortized cost, 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	6.8	22.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.9	3.0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	3.4	5.3
Total Unrealized Capital Losses	11.1	31.1
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	558.1	717.7
Total Unrealized Capital Losses	9.1	28.8
More than twelve months below amortized cost, Number of Securities	82	119
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	4.7	7.9
Total Unrealized Capital Losses	2.0	2.3
More than twelve months below amortized cost, Number of Securities	2	3
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	4.7	19.1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.5	4.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.2	17.8
Total Unrealized Capital Losses	16.4	41.5
Foreign | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	242.7	670.5
Total Unrealized Capital Losses	5.7	31.9
More than twelve months below amortized cost, Number of Securities	38	122
Foreign | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	36.0	25.7
Total Unrealized Capital Losses	10.7	9.6
More than twelve months below amortized cost, Number of Securities	8	7
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	1.6	1.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0.3	5.2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	16.2	46.1
Total Unrealized Capital Losses	18.1	52.9
Residential mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	201.2	276.5
Total Unrealized Capital Losses	10.2	19.0
More than twelve months below amortized cost, Number of Securities	124	119
Residential mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	25.2	98.2
Total Unrealized Capital Losses	7.9	33.9
More than twelve months below amortized cost, Number of Securities	24	47
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0.1	1.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	2.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	0.1	1.5
Total Unrealized Capital Losses	0.2	5.8
Commercial mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	11.8	110.1
Total Unrealized Capital Losses	0.2	5.4
More than twelve months below amortized cost, Number of Securities	8	16
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	1.9
Total Unrealized Capital Losses	0	0.4
More than twelve months below amortized cost, Number of Securities	0	1
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0.2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	1.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.7	20.6
Total Unrealized Capital Losses	6.7	22.1
Other asset-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	44.1	48.3
Total Unrealized Capital Losses	2.4	2.4
More than twelve months below amortized cost, Number of Securities	17	61
Other asset-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	12.4	55.4
Total Unrealized Capital Losses	$ 4.3	$ 19.7
More than twelve months below amortized cost, Number of Securities	6	19