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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Jun. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 3,181.3	$ 1,965.3
0-12 months, Gross Unrealized Loss	(60.3)	(12.0)
Over 12 months, Fair Market Value	916.7	984.3
Over 12 months, Gross Unrealized Loss	(39.1)	(21.9)
Total, Fair Market Value	4,098.0	2,949.6
Total, Gross Unrealized Loss	$ (99.4)	$ (33.9)
Number of Securities | Security	1,157	769
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 3,139.8	$ 1,918.4
0-12 months, Gross Unrealized Loss	(60.3)	(11.9)
Over 12 months, Fair Market Value	916.7	984.3
Over 12 months, Gross Unrealized Loss	(39.1)	(21.9)
Total, Fair Market Value	4,056.5	2,902.7
Total, Gross Unrealized Loss	$ (99.4)	$ (33.8)
Number of Securities | Security	1,149	761
Total short-term investments — Trading
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 41.5	$ 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	41.5	46.9
Total, Gross Unrealized Loss	$ 0.0	$ (0.1)
Number of Securities | Security	8	8
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 808.1	$ 652.1
0-12 months, Gross Unrealized Loss	(15.8)	(5.1)
Over 12 months, Fair Market Value	256.1	259.8
Over 12 months, Gross Unrealized Loss	(9.7)	(6.5)
Total, Fair Market Value	1,064.2	911.9
Total, Gross Unrealized Loss	$ (25.5)	$ (11.6)
Number of Securities | Security	123	101
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 32.9	$ 20.1
0-12 months, Gross Unrealized Loss	(0.6)	(0.2)
Over 12 months, Fair Market Value	0.0	6.1
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	32.9	26.2
Total, Gross Unrealized Loss	$ (0.6)	$ (0.2)
Number of Securities | Security	13	10
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 90.0	$ 28.5
0-12 months, Gross Unrealized Loss	(1.1)	(0.2)
Over 12 months, Fair Market Value	0.7	0.0
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	90.7	28.5
Total, Gross Unrealized Loss	$ (1.1)	$ (0.2)
Number of Securities | Security	13	9
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,556.6	$ 699.3
0-12 months, Gross Unrealized Loss	(31.9)	(3.4)
Over 12 months, Fair Market Value	285.3	360.7
Over 12 months, Gross Unrealized Loss	(15.4)	(8.4)
Total, Fair Market Value	1,841.9	1,060.0
Total, Gross Unrealized Loss	$ (47.3)	$ (11.8)
Number of Securities | Security	682	412
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 42.9	$ 43.5
0-12 months, Gross Unrealized Loss	(0.2)	(0.3)
Over 12 months, Fair Market Value	20.9	13.3
Over 12 months, Gross Unrealized Loss	(0.3)	(0.2)
Total, Fair Market Value	63.8	56.8
Total, Gross Unrealized Loss	$ (0.5)	$ (0.5)
Number of Securities | Security	14	15
Non-U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 188.9	$ 206.2
0-12 months, Gross Unrealized Loss	(0.8)	(0.8)
Over 12 months, Fair Market Value	59.3	32.0
Over 12 months, Gross Unrealized Loss	(0.5)	(0.4)
Total, Fair Market Value	248.2	238.2
Total, Gross Unrealized Loss	$ (1.3)	$ (1.2)
Number of Securities | Security	57	47
Asset-backed
Schedule of Available-for-sale Securities [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	(0.1)	(0.1)
Total, Fair Market Value	18.4	21.6
Total, Gross Unrealized Loss	$ (0.3)	$ (0.1)
Number of Securities | Security	7	11
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 407.0	$ 257.6
0-12 months, Gross Unrealized Loss	(9.7)	(1.9)
Over 12 months, Fair Market Value	289.4	301.9
Over 12 months, Gross Unrealized Loss	(13.1)	(6.3)
Total, Fair Market Value	696.4	559.5
Total, Gross Unrealized Loss	$ (22.8)	$ (8.2)
Number of Securities | Security	240	156

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