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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Sep. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 3,106.3	$ 1,965.3
0-12 months, Gross Unrealized Loss	(57.1)	(12.0)
Over 12 months, Fair Market Value	1,334.8	984.3
Over 12 months, Gross Unrealized Loss	(58.0)	(21.9)
Total, Fair Market Value	4,441.1	2,949.6
Total, Gross Unrealized Loss	$ (115.1)	$ (33.9)
Number of Securities | Security	1,254	769
Total fixed income securities — Available for sale
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 3,029.9	$ 1,918.4
0-12 months, Gross Unrealized Loss	(57.1)	(11.9)
Over 12 months, Fair Market Value	1,334.8	984.3
Over 12 months, Gross Unrealized Loss	(58.0)	(21.9)
Total, Fair Market Value	4,364.7	2,902.7
Total, Gross Unrealized Loss	$ (115.1)	$ (33.8)
Number of Securities | Security	1,241	761
Total short-term investments — Trading
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 76.4	$ 46.9
0-12 months, Gross Unrealized Loss	0.0	(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	76.4	46.9
Total, Gross Unrealized Loss	$ 0.0	$ (0.1)
Number of Securities | Security	13	8
U.S. government
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 827.9	$ 652.1
0-12 months, Gross Unrealized Loss	(15.3)	(5.1)
Over 12 months, Fair Market Value	457.3	259.8
Over 12 months, Gross Unrealized Loss	(17.8)	(6.5)
Total, Fair Market Value	1,285.2	911.9
Total, Gross Unrealized Loss	$ (33.1)	$ (11.6)
Number of Securities | Security	140	101
U.S. agency
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 25.5	$ 20.1
0-12 months, Gross Unrealized Loss	(0.4)	(0.2)
Over 12 months, Fair Market Value	13.1	6.1
Over 12 months, Gross Unrealized Loss	(0.4)	0.0
Total, Fair Market Value	38.6	26.2
Total, Gross Unrealized Loss	$ (0.8)	$ (0.2)
Number of Securities | Security	15	10
Municipal
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 33.3	$ 28.5
0-12 months, Gross Unrealized Loss	(1.1)	(0.2)
Over 12 months, Fair Market Value	7.2	0.0
Over 12 months, Gross Unrealized Loss	(0.4)	0.0
Total, Fair Market Value	40.5	28.5
Total, Gross Unrealized Loss	$ (1.5)	$ (0.2)
Number of Securities | Security	13	9
Corporate
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 1,466.5	$ 699.3
0-12 months, Gross Unrealized Loss	(28.7)	(3.4)
Over 12 months, Fair Market Value	416.7	360.7
Over 12 months, Gross Unrealized Loss	(19.2)	(8.4)
Total, Fair Market Value	1,883.2	1,060.0
Total, Gross Unrealized Loss	$ (47.9)	$ (11.8)
Number of Securities | Security	725	412
Non-U.S. government-backed corporate
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 43.2	$ 43.5
0-12 months, Gross Unrealized Loss	(0.3)	(0.3)
Over 12 months, Fair Market Value	25.4	13.3
Over 12 months, Gross Unrealized Loss	(0.4)	(0.2)
Total, Fair Market Value	68.6	56.8
Total, Gross Unrealized Loss	$ (0.7)	$ (0.5)
Number of Securities | Security	15	15
Non-U.S. government
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 193.8	$ 206.2
0-12 months, Gross Unrealized Loss	(0.9)	(0.8)
Over 12 months, Fair Market Value	50.1	32.0
Over 12 months, Gross Unrealized Loss	(0.9)	(0.4)
Total, Fair Market Value	243.9	238.2
Total, Gross Unrealized Loss	$ (1.8)	$ (1.2)
Number of Securities | Security	61	47
Asset-backed
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 13.4	$ 11.1
0-12 months, Gross Unrealized Loss	(0.2)	0.0
Over 12 months, Fair Market Value	5.0	10.5
Over 12 months, Gross Unrealized Loss	0.0	(0.1)
Total, Fair Market Value	18.4	21.6
Total, Gross Unrealized Loss	$ (0.2)	$ (0.1)
Number of Securities | Security	7	11
Agency mortgage-backed
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 426.3	$ 257.6
0-12 months, Gross Unrealized Loss	(10.2)	(1.9)
Over 12 months, Fair Market Value	360.0	301.9
Over 12 months, Gross Unrealized Loss	(18.9)	(6.3)
Total, Fair Market Value	786.3	559.5
Total, Gross Unrealized Loss	$ (29.1)	$ (8.2)
Number of Securities | Security	265	156

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