Fair Value Measurements (Details 2) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Available-for-sale | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	$ 412.2	$ 336.8
Total realized/unrealized gains (losses) included in net income, assets		(0.8)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	4.4	1.3
Net purchases, sales, issuances and settlements, assets	22.5	(3.0)
Transfers into Level 3, assets	19.5	6.6
Transfers out of Level 3, assets	(62.1)	(37.2)
Ending balance, assets	396.5	303.7
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.1)	(0.9)
Gross purchases, sales, issuances and settlements
Purchases, assets	39.1	10.8
Sales, assets	(3.6)	(5.9)
Settlements, assets	(13.0)	(7.9)
Net purchases, sales, issuances and settlements, assets	22.5	(3.0)
Available-for-sale | Non-U.S. governments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	38.7	47.5
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.3	(0.3)
Net purchases, sales, issuances and settlements, assets	1.6	2.9
Ending balance, assets	40.6	50.1
Gross purchases, sales, issuances and settlements
Purchases, assets	1.9	3.2
Settlements, assets	(0.3)	(0.3)
Net purchases, sales, issuances and settlements, assets	1.6	2.9
Available-for-sale | States and political subdivisions
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets			1.8
Ending balance, assets		1.8	1.8
Available-for-sale | Corporate debt securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	245.6	164.0
Total realized/unrealized gains (losses) included in net income, assets	(0.1)	(0.1)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	3.7	(0.1)
Net purchases, sales, issuances and settlements, assets	16.0	1.7
Transfers into Level 3, assets	19.5	0.5
Transfers out of Level 3, assets	(47.7)
Ending balance, assets	237.0	166.0
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.1)	(0.1)
Gross purchases, sales, issuances and settlements
Purchases, assets	27.1	7.6
Sales, assets	(3.6)	(5.9)
Settlements, assets	(7.5)
Net purchases, sales, issuances and settlements, assets	16.0	1.7
Available-for-sale | Commercial mortgage-backed securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets		1.6
Total realized/unrealized gains (losses) included in net income, assets		(0.7)
Total realized/unrealized gains (losses) included in other comprehensive income, assets		1.2
Net purchases, sales, issuances and settlements, assets		(0.5)
Transfers into Level 3, assets		6.1
Ending balance, assets		7.7
Changes in unrealized gains (losses) included in net income relating to positions still held, assets		(0.8)
Gross purchases, sales, issuances and settlements
Settlements, assets		(0.5)
Net purchases, sales, issuances and settlements, assets		(0.5)
Available-for-sale | Collateralized debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	64.2	37.8
Total realized/unrealized gains (losses) included in net income, assets	0.1
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.1)	0.1
Net purchases, sales, issuances and settlements, assets	(0.6)
Transfers out of Level 3, assets		(7.5)
Ending balance, assets	63.6	30.4
Gross purchases, sales, issuances and settlements
Settlements, assets	(0.6)
Net purchases, sales, issuances and settlements, assets	(0.6)
Available-for-sale | Other debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	63.7	84.1
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.5	0.4
Net purchases, sales, issuances and settlements, assets	5.5	(7.1)
Transfers out of Level 3, assets	(14.4)	(29.7)
Ending balance, assets	55.3	47.7
Gross purchases, sales, issuances and settlements
Purchases, assets	10.1
Settlements, assets	(4.6)	(7.1)
Net purchases, sales, issuances and settlements, assets	5.5	(7.1)
Available-for-sale | Equity securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets		16.9		4.1
Total realized/unrealized gains (losses) included in other comprehensive income, assets		0.7
Ending balance, assets	4.1	17.6		4.1
Trading | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	139.7	169.9
Total realized/unrealized gains (losses) included in net income, assets	0.7	4.7
Net purchases, sales, issuances and settlements, assets	(0.2)
Ending balance, assets	140.2	174.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	0.6	4.6
Gross purchases, sales, issuances and settlements
Sales, assets	(0.2)
Net purchases, sales, issuances and settlements, assets	(0.2)
Derivative assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	53.7	74.2
Total realized/unrealized gains (losses) included in net income, assets	4.0	(9.1)
Net purchases, sales, issuances and settlements, assets	2.5
Ending balance, assets	60.2	65.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	4.2	(9.1)
Gross purchases, sales, issuances and settlements
Purchases, assets	2.5
Net purchases, sales, issuances and settlements, assets	2.5
Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	127.2	142.9
Total realized/unrealized gains (losses) included in net income, assets	3.8	(0.4)
Net purchases, sales, issuances and settlements, assets	(67.4)	(8.9)
Transfers out of Level 3, assets	(35.0)
Ending balance, assets	28.6	133.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	3.8	(0.5)
Gross purchases, sales, issuances and settlements
Purchases, assets	0.3
Sales, assets	(67.7)
Settlements, assets		(8.9)
Net purchases, sales, issuances and settlements, assets	(67.4)	(8.9)
Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	5,891.4	5,265.2
Total realized/unrealized gains (losses) included in net income, assets	215.7	102.2
Total realized/unrealized gains (losses) included in other comprehensive income, assets		(0.1)
Net purchases, sales, issuances and settlements, assets	(76.6)	(22.5)
Transfers into Level 3, assets	0.1	2.0
Transfers out of Level 3, assets		(5.8)
Ending balance, assets	6,030.6	5,341.0
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	198.5	102.2
Gross purchases, sales, issuances and settlements
Purchases, assets	139.2	78.3
Sales, assets	(164.7)	(80.2)
Issuances, assets	(49.3)	(53.4)
Settlements, assets	(1.8)	32.8
Net purchases, sales, issuances and settlements, assets	(76.6)	(22.5)
Investment-type insurance contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(176.4)	(6.9)
Total realized/unrealized gains (losses) included in net income, liabilities	(51.0)	(27.5)
Net purchases, sales, issuances and settlements, liabilities	(1.3)	8.6
Ending balance, liabilities	(228.7)	(25.8)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(52.1)	(27.4)
Gross purchases, sales, issuances and settlements
Issuances, liabilities	(3.1)	7.1
Settlements, liabilities	1.8	1.5
Net purchases, sales, issuances and settlements, liabilities	(1.3)	8.6
Derivative liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(35.5)	(39.6)
Total realized/unrealized gains (losses) included in net income, liabilities	(15.8)	4.9
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	0.3	0.3
Net purchases, sales, issuances and settlements, liabilities	0.2
Ending balance, liabilities	(50.8)	(34.4)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(15.8)	4.9
Gross purchases, sales, issuances and settlements
Sales, liabilities	0.2
Net purchases, sales, issuances and settlements, liabilities	0.2
Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(66.3)	(73.9)
Total realized/unrealized gains (losses) included in net income, liabilities	1.0	(2.0)
Ending balance, liabilities	(65.3)	(75.9)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	$ 0.9	$ (2.0)