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Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Trading Securities, Realized Gain (Loss)	$ 279.9	$ 76.9	$ 166.8
Foreign Currency Transaction Gain (Loss), Realized	0.4	0.4	0.0
Net realized investment gains, pre-tax	280.3	77.3	166.8
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(49.5)	(22.8)	(28.4)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	49.5	(22.8)	(28.4)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax	230.4	54.1	138.4
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax	0.4	0.4	0.0
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	230.8	54.5	138.4
Fixed maturity investments
Investment securities
Trading Securities, Realized Gain (Loss)	(2.1)	1.9	5.8
Foreign Currency Transaction Gain (Loss), Realized	0.4	0.0	0.0
Net realized investment gains, pre-tax	(1.7)	1.9	5.8
Short-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	0.4
Foreign Currency Transaction Gain (Loss), Realized	0.0
Net realized investment gains, pre-tax	0.4
Common equity securities
Investment securities
Trading Securities, Realized Gain (Loss)	280.7	64.4	138.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.4	0.0
Net realized investment gains, pre-tax	280.7	64.8	138.0
Other long-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	0.9	10.6	23.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0
Net realized investment gains, pre-tax	$ 0.9	$ 10.6	$ 23.0

X

- Definition

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

+ References

Reference 1: 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 2: 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=64836518&loc=d3e30690-110894

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64836518&loc=d3e30700-110894

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

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