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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Mar. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 3,164.2	$ 1,965.3
0-12 months, Gross Unrealized Loss	(50.6)	(12.0)
Over 12 months, Fair Market Value	942.0	984.3
Over 12 months, Gross Unrealized Loss	(36.3)	(21.9)
Total, Fair Market Value	4,106.2	2,949.6
Total, Gross Unrealized Loss	$ (86.9)	$ (33.9)
Number of Securities | Security	1,116	769
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 3,122.4	$ 1,918.4
0-12 months, Gross Unrealized Loss	(50.5)	(11.9)
Over 12 months, Fair Market Value	942.0	984.3
Over 12 months, Gross Unrealized Loss	(36.3)	(21.9)
Total, Fair Market Value	4,064.4	2,902.7
Total, Gross Unrealized Loss	$ (86.8)	$ (33.8)
Number of Securities | Security	1,106	761
Total short-term investments — Trading
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 41.8	$ 46.9
0-12 months, Gross Unrealized Loss	(0.1)	(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	41.8	46.9
Total, Gross Unrealized Loss	$ (0.1)	$ (0.1)
Number of Securities | Security	10	8
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 772.9	$ 652.1
0-12 months, Gross Unrealized Loss	(12.7)	(5.1)
Over 12 months, Fair Market Value	286.8	259.8
Over 12 months, Gross Unrealized Loss	(8.9)	(6.5)
Total, Fair Market Value	1,059.7	911.9
Total, Gross Unrealized Loss	$ (21.6)	$ (11.6)
Number of Securities | Security	112	101
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 30.8	$ 20.1
0-12 months, Gross Unrealized Loss	(0.4)	(0.2)
Over 12 months, Fair Market Value	5.5	6.1
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	36.3	26.2
Total, Gross Unrealized Loss	$ (0.4)	$ (0.2)
Number of Securities | Security	12	10
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 36.0	$ 28.5
0-12 months, Gross Unrealized Loss	(0.6)	(0.2)
Over 12 months, Fair Market Value	0.0	0.0
Over 12 months, Gross Unrealized Loss	0.0	0.0
Total, Fair Market Value	36.0	28.5
Total, Gross Unrealized Loss	$ (0.6)	$ (0.2)
Number of Securities | Security	11	9
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,544.3	$ 699.3
0-12 months, Gross Unrealized Loss	(26.3)	(3.4)
Over 12 months, Fair Market Value	320.4	360.7
Over 12 months, Gross Unrealized Loss	(15.0)	(8.4)
Total, Fair Market Value	1,864.7	1,060.0
Total, Gross Unrealized Loss	$ (41.3)	$ (11.8)
Number of Securities | Security	661	412
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 50.0	$ 43.5
0-12 months, Gross Unrealized Loss	(0.4)	(0.3)
Over 12 months, Fair Market Value	10.0	13.3
Over 12 months, Gross Unrealized Loss	(0.1)	(0.2)
Total, Fair Market Value	60.0	56.8
Total, Gross Unrealized Loss	$ (0.5)	$ (0.5)
Number of Securities | Security	17	15
Non-U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 247.5	$ 206.2
0-12 months, Gross Unrealized Loss	(1.4)	(0.8)
Over 12 months, Fair Market Value	23.8	32.0
Over 12 months, Gross Unrealized Loss	(0.4)	(0.4)
Total, Fair Market Value	271.3	238.2
Total, Gross Unrealized Loss	$ (1.8)	$ (1.2)
Number of Securities | Security	58	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	(0.1)	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	$ (0.2)	$ (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	$ 427.5	$ 257.6
0-12 months, Gross Unrealized Loss	(8.6)	(1.9)
Over 12 months, Fair Market Value	290.5	301.9
Over 12 months, Gross Unrealized Loss	(11.8)	(6.3)
Total, Fair Market Value	718.0	559.5
Total, Gross Unrealized Loss	$ (20.4)	$ (8.2)
Number of Securities | Security	228	156

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