Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment securities
Trading Securities, Realized Gain (Loss)	$ 208.7	$ 121.8	$ 76.7
Foreign Currency Transaction Gain (Loss), Realized	25.1	(17.3)	(8.6)
Net realized investment gains, pre-tax	233.8	104.5	68.1
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(39.7)	(30.0)	(23.8)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	(8.0)	5.5	2.2
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(47.7)	(24.5)	(21.6)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax	169.0	91.8	52.9
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax	17.1	(11.8)	(6.4)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	186.1	80.0	46.5
Fixed maturity investments
Investment securities
Trading Securities, Realized Gain (Loss)	23.9	8.6	82.5
Foreign Currency Transaction Gain (Loss), Realized	21.0	(15.0)	(2.5)
Net realized investment gains, pre-tax	44.9	(6.4)	80.0
Short-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	0	0.1	0
Foreign Currency Transaction Gain (Loss), Realized	2.9	0	(4.3)
Net realized investment gains, pre-tax	2.9	0.1	(4.3)
Common equity securities
Investment securities
Trading Securities, Realized Gain (Loss)	150.9	104.5	1.3
Foreign Currency Transaction Gain (Loss), Realized	0.9	(3.7)	0
Net realized investment gains, pre-tax	151.8	100.8	1.3
Convertible fixed maturity and preferred investments
Investment securities
Trading Securities, Realized Gain (Loss)	5.9	1.0	(8.2)
Foreign Currency Transaction Gain (Loss), Realized	0	0	(1.8)
Net realized investment gains, pre-tax	5.9	1.0	(10.0)
Other long-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	27.7	7.5	1.4
Foreign Currency Transaction Gain (Loss), Realized	0.3	1.4	0
Net realized investment gains, pre-tax	28.0	8.9	1.4
Forward contracts
Investment securities
Trading Securities, Realized Gain (Loss)	0.3	0.1	(0.3)
Foreign Currency Transaction Gain (Loss), Realized	0	0	0
Net realized investment gains, pre-tax	$ 0.3	$ 0.1	$ (0.3)

Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 830

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6450189&loc=d3e30690-110894

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 830

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 830

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph c

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

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