Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Jun. 30, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 2,117.9	$ 2,518.2
0-12 months, Gross Unrealized Loss	(24.6)	(39.3)
Over 12 months, Fair Market Value	49.0	37.0
Over 12 months, Gross Unrealized Loss	(0.8)	(1.5)
Total, Fair Market Value	2,166.9	2,555.2
Total, Gross Unrealized Loss	$ (25.4)	$ (40.8)
Number of Securities \| Security	563	679
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 2,092.5	$ 2,517.1
0-12 months, Gross Unrealized Loss	(24.6)	(39.3)
Over 12 months, Fair Market Value	49.0	37.0
Over 12 months, Gross Unrealized Loss	(0.8)	(1.5)
Total, Fair Market Value	2,141.5	2,554.1
Total, Gross Unrealized Loss	$ (25.4)	$ (40.8)
Number of Securities \| Security	554	677
Total short-term investments — Trading
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 25.4	$ 1.1
0-12 months, Gross Unrealized Loss	0.0	0.0
Over 12 months, Fair Market Value	0.0	0.0
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	25.4	1.1
Total, Gross Unrealized Loss	$ 0.0	$ 0.0
Number of Securities \| Security	9	2
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 663.7	$ 724.4
0-12 months, Gross Unrealized Loss	(6.7)	(11.2)
Over 12 months, Fair Market Value	0.0	0.0
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	663.7	724.4
Total, Gross Unrealized Loss	$ (6.7)	$ (11.2)
Number of Securities \| Security	76	78
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 17.8	$ 14.1
0-12 months, Gross Unrealized Loss	0.0	0.0
Over 12 months, Fair Market Value	0.0	0.0
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	17.8	14.1
Total, Gross Unrealized Loss	$ 0.0	$ 0.0
Number of Securities \| Security	6	4
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 10.9	$ 7.7
0-12 months, Gross Unrealized Loss	0.0	(0.2)
Over 12 months, Fair Market Value	0.0	0.8
Over 12 months, Gross Unrealized Loss	0.0	(0.2)
Total, Fair Market Value	10.9	8.5
Total, Gross Unrealized Loss	$ 0.0	$ (0.4)
Number of Securities \| Security	4	6
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 693.8	$ 1,044.4
0-12 months, Gross Unrealized Loss	(10.5)	(19.4)
Over 12 months, Fair Market Value	3.4	6.6
Over 12 months, Gross Unrealized Loss	(0.1)	(0.6)
Total, Fair Market Value	697.2	1,051.0
Total, Gross Unrealized Loss	$ (10.6)	$ (20.0)
Number of Securities \| Security	274	386
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 54.0	$ 29.6
0-12 months, Gross Unrealized Loss	(0.2)	(0.1)
Over 12 months, Fair Market Value	0.0	0.0
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	54.0	29.6
Total, Gross Unrealized Loss	$ (0.2)	$ (0.1)
Number of Securities \| Security	14	11
Non-U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 150.7	$ 143.5
0-12 months, Gross Unrealized Loss	(0.7)	(0.8)
Over 12 months, Fair Market Value	21.0	1.0
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	171.7	144.5
Total, Gross Unrealized Loss	$ (0.7)	$ (0.8)
Number of Securities \| Security	30	29
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 13.4	$ 25.8
0-12 months, Gross Unrealized Loss	0.0	0.0
Over 12 months, Fair Market Value	0.2	1.4
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	13.6	27.2
Total, Gross Unrealized Loss	$ 0.0	$ 0.0
Number of Securities \| Security	11	15
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 488.2	$ 527.6
0-12 months, Gross Unrealized Loss	(6.5)	(7.6)
Over 12 months, Fair Market Value	24.4	27.2
Over 12 months, Gross Unrealized Loss	(0.7)	(0.7)
Total, Fair Market Value	512.6	554.8
Total, Gross Unrealized Loss	$ (7.2)	$ (8.3)
Number of Securities \| Security	139	148

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_ForeignCorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: