Investments - Unrealized Capital Losses 1 (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 263.7	$ 13.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	37.6	1.7
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	25.7	37.6
Total Unrealized Capital Losses	327.0	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	5,267.8	1,110.8
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	265.2	19.3
Six months or less below amortized cost, Number of Securities	712	141
More than six months and twelve months or less below amortized cost, Amortized Cost	406.8	49.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	39.7	2.6
More than six months and twelve months or less below amortized cost, Number of Securities	71	31
More than twelve months below amortized cost, Amortized Cost	103.3	198.1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.1	6.2
More than twelve months below amortized cost, Number of Securities	98	99
Total Amortized Cost	5,777.9	1,358.4
Total Unrealized Capital Losses	311.0	28.1
Total Number of Securities	881	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	40.9	15.2
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	9.4	3.9
Six months or less below amortized cost, Number of Securities	12	10
More than six months and twelve months or less below amortized cost, Amortized Cost	4.5	1.5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	1.1	0.4
More than six months and twelve months or less below amortized cost, Number of Securities	4	2
More than twelve months below amortized cost, Amortized Cost	20.6	61.6
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5.5	20.6
More than twelve months below amortized cost, Number of Securities	9	28
Total Amortized Cost	66.0	78.3
Total Unrealized Capital Losses	16.0	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	4.8	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	4.8	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	221.8	300.5
Total Unrealized Capital Losses	4.8	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	159.3	6.8
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	29.1	0.9
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	4.1	3.4
Total Unrealized Capital Losses	192.5	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	3,355.2	558.1
Total Unrealized Capital Losses	186.0	9.1
Total Number of Securities	452	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	28.4	4.7
Total Unrealized Capital Losses	6.5	2.0
Total Number of Securities	4	2
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	88.8	4.7
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	5.8	0.5
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	6.1	11.2
Total Unrealized Capital Losses	100.7	16.4
Foreign | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	1,595.5	242.7
Total Unrealized Capital Losses	97.3	5.7
Total Number of Securities	231	38
Foreign | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	14.7	36.0
Total Unrealized Capital Losses	3.4	10.7
Total Number of Securities	3	8
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Loss	10.6	1.6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	2.7	0.3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	11.7	16.2
Total Unrealized Capital Losses	25.0	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	510.0	201.2
Total Unrealized Capital Losses	20.3	10.2
Total Number of Securities	169	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	18.1	25.2
Total Unrealized Capital Losses	4.7	7.9
Total Number of Securities	14	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	0.1	0.1
Total Unrealized Capital Losses	0.1	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	2.2	11.8
Total Unrealized Capital Losses	0.1	0.2
Total Number of Securities	2	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	3.7	6.7
Total Unrealized Capital Losses	3.9	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	93.2	44.1
Total Unrealized Capital Losses	2.5	2.4
Total Number of Securities	24	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	4.8	12.4
Total Unrealized Capital Losses	$ 1.4	$ 4.3
Total Number of Securities	4	6