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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Mar. 31, 2019
0-12 months, Fair Market Value	$ 171.5	$ 1,560.9
0-12 months, Gross Unrealized Loss	(0.4)	(19.1)
Over 12 months, Fair Market Value	2,243.2	2,121.1
Over 12 months, Gross Unrealized Loss	(29.4)	(58.8)
Total, Fair Market Value	2,414.7	3,682.0
Total, Gross Unrealized Loss	$ (29.8)	$ (77.9)
Number of Securities | Security	725	1,133
Total fixed income securities — Available for sale
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 154.3	$ 1,526.4
0-12 months, Gross Unrealized Loss	(0.4)	(19.1)
Over 12 months, Fair Market Value	2,243.2	2,121.1
Over 12 months, Gross Unrealized Loss	(29.4)	(58.8)
Total, Fair Market Value	2,397.5	3,647.5
Total, Gross Unrealized Loss	$ (29.8)	$ (77.9)
Number of Securities | Security	718	1,121
Total short-term investments — Trading
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 17.2	$ 34.5
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	17.2	34.5
Total, Gross Unrealized Loss	$ 0.0	$ 0.0
Number of Securities | Security	7	12
U.S. government
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 90.9	$ 180.2
0-12 months, Gross Unrealized Loss	(0.2)	(0.7)
Over 12 months, Fair Market Value	645.1	740.6
Over 12 months, Gross Unrealized Loss	(7.8)	(15.4)
Total, Fair Market Value	736.0	920.8
Total, Gross Unrealized Loss	$ (8.0)	$ (16.1)
Number of Securities | Security	75	103
U.S. agency
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 1.4	$ 13.5
0-12 months, Gross Unrealized Loss	0.0	(0.2)
Over 12 months, Fair Market Value	14.8	18.4
Over 12 months, Gross Unrealized Loss	(0.1)	(0.2)
Total, Fair Market Value	16.2	31.9
Total, Gross Unrealized Loss	$ (0.1)	$ (0.4)
Number of Securities | Security	7	12
Municipal
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 0.0	$ 3.1
0-12 months, Gross Unrealized Loss	0.0	(0.1)
Over 12 months, Fair Market Value	25.4	25.0
Over 12 months, Gross Unrealized Loss	(0.2)	(0.7)
Total, Fair Market Value	25.4	28.1
Total, Gross Unrealized Loss	$ (0.2)	$ (0.8)
Number of Securities | Security	8	9
Corporate
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 27.8	$ 999.1
0-12 months, Gross Unrealized Loss	(0.1)	(15.2)
Over 12 months, Fair Market Value	966.9	762.2
Over 12 months, Gross Unrealized Loss	(10.7)	(25.3)
Total, Fair Market Value	994.7	1,761.3
Total, Gross Unrealized Loss	$ (10.8)	$ (40.5)
Number of Securities | Security	380	667
Non-U.S. government-backed corporate
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 0.0	$ 14.5
0-12 months, Gross Unrealized Loss	0.0	0.0
Over 12 months, Fair Market Value	28.1	25.8
Over 12 months, Gross Unrealized Loss	(0.2)	(0.2)
Total, Fair Market Value	28.1	40.3
Total, Gross Unrealized Loss	$ (0.2)	$ (0.2)
Number of Securities | Security	10	12
Non-U.S. government
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 5.7	$ 64.0
0-12 months, Gross Unrealized Loss	0.0	(0.3)
Over 12 months, Fair Market Value	53.2	91.0
Over 12 months, Gross Unrealized Loss	(0.2)	(0.5)
Total, Fair Market Value	58.9	155.0
Total, Gross Unrealized Loss	$ (0.2)	$ (0.8)
Number of Securities | Security	35	57
Asset-backed
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 0.0	$ 6.3
0-12 months, Gross Unrealized Loss	0.0	0.0
Over 12 months, Fair Market Value	13.1	10.8
Over 12 months, Gross Unrealized Loss	(0.1)	(0.1)
Total, Fair Market Value	13.1	17.1
Total, Gross Unrealized Loss	$ (0.1)	$ (0.1)
Number of Securities | Security	6	8
Agency mortgage-backed
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 28.5	$ 245.7
0-12 months, Gross Unrealized Loss	(0.1)	(2.6)
Over 12 months, Fair Market Value	496.6	447.3
Over 12 months, Gross Unrealized Loss	(10.1)	(16.4)
Total, Fair Market Value	525.1	693.0
Total, Gross Unrealized Loss	$ (10.2)	$ (19.0)
Number of Securities | Security	197	253

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