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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Dec. 31, 2017 USD ($) security Security	Dec. 31, 2016 USD ($) security Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,965.3	$ 2,518.2
0-12 months, Gross Unrealized Losses	(12.0)	(39.3)
Over 12 months, Fair Market Value	984.3	37.0
Over 12 months, Gross Unrealized Losses	(21.9)	(1.5)
Total, Fair Market Value	2,949.6	2,555.2
Total, Gross Unrealized Losses	$ (33.9)	$ (40.8)
Number of Securities | security	769	679
Fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,918.4	$ 2,517.1
0-12 months, Gross Unrealized Losses	(11.9)	(39.3)
Over 12 months, Fair Market Value	984.3	37.0
Over 12 months, Gross Unrealized Losses	(21.9)	(1.5)
Total, Fair Market Value	2,902.7	2,554.1
Total, Gross Unrealized Losses	$ (33.8)	$ (40.8)
Number of Securities | Security	761	677
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 46.9	$ 1.1
0-12 months, Gross Unrealized Losses	(0.1)	0.0
Over 12 months, Fair Market Value	0.0	0.0
Over 12 months, Gross Unrealized Losses	0.0	0.0
Total, Fair Market Value	46.9	1.1
Total, Gross Unrealized Losses	$ (0.1)	$ 0.0
Number of Securities | Security	8	2
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 652.1	$ 724.4
0-12 months, Gross Unrealized Losses	(5.1)	(11.2)
Over 12 months, Fair Market Value	259.8	0.0
Over 12 months, Gross Unrealized Losses	(6.5)	0.0
Total, Fair Market Value	911.9	724.4
Total, Gross Unrealized Losses	$ (11.6)	$ (11.2)
Number of Securities | security	101	78
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 20.1	$ 14.1
0-12 months, Gross Unrealized Losses	(0.2)	0.0
Over 12 months, Fair Market Value	6.1	0.0
Over 12 months, Gross Unrealized Losses	0.0	0.0
Total, Fair Market Value	26.2	14.1
Total, Gross Unrealized Losses	$ (0.2)	$ 0.0
Number of Securities | security	10	4
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 28.5	$ 7.7
0-12 months, Gross Unrealized Losses	(0.2)	(0.2)
Over 12 months, Fair Market Value	0.0	0.8
Over 12 months, Gross Unrealized Losses	0.0	(0.2)
Total, Fair Market Value	28.5	8.5
Total, Gross Unrealized Losses	$ (0.2)	$ (0.4)
Number of Securities | security	9	6
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 699.3	$ 1,044.4
0-12 months, Gross Unrealized Losses	(3.4)	(19.4)
Over 12 months, Fair Market Value	360.7	6.6
Over 12 months, Gross Unrealized Losses	(8.4)	(0.6)
Total, Fair Market Value	1,060.0	1,051.0
Total, Gross Unrealized Losses	$ (11.8)	$ (20.0)
Number of Securities | security	412	386
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 43.5	$ 29.6
0-12 months, Gross Unrealized Losses	(0.3)	(0.1)
Over 12 months, Fair Market Value	13.3	0.0
Over 12 months, Gross Unrealized Losses	(0.2)	0.0
Total, Fair Market Value	56.8	29.6
Total, Gross Unrealized Losses	$ (0.5)	$ (0.1)
Number of Securities | security	15	11
Non-U.S government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 206.2	$ 143.5
0-12 months, Gross Unrealized Losses	(0.8)	(0.8)
Over 12 months, Fair Market Value	32.0	1.0
Over 12 months, Gross Unrealized Losses	(0.4)	0.0
Total, Fair Market Value	238.2	144.5
Total, Gross Unrealized Losses	$ (1.2)	$ (0.8)
Number of Securities | security	47	29
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 11.1	$ 25.8
0-12 months, Gross Unrealized Losses	0.0	0.0
Over 12 months, Fair Market Value	10.5	1.4
Over 12 months, Gross Unrealized Losses	(0.1)	0.0
Total, Fair Market Value	21.6	27.2
Total, Gross Unrealized Losses	$ (0.1)	$ 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	$ 257.6	$ 527.6
0-12 months, Gross Unrealized Losses	(1.9)	(7.6)
Over 12 months, Fair Market Value	301.9	27.2
Over 12 months, Gross Unrealized Losses	(6.3)	(0.7)
Total, Fair Market Value	559.5	554.8
Total, Gross Unrealized Losses	$ (8.2)	$ (8.3)
Number of Securities | security	156	148

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