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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Dec. 31, 2018 USD ($) security Security	Dec. 31, 2017 USD ($) security Security
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 1,560.9	$ 1,965.3
0-12 months, Gross Unrealized Losses	(19.1)	(12.0)
Over 12 months, Fair Market Value	2,121.1	984.3
Over 12 months, Gross Unrealized Losses	(58.8)	(21.9)
Total, Fair Market Value	3,682.0	2,949.6
Total, Gross Unrealized Losses	$ (77.9)	$ (33.9)
Number of Securities | security	1,133	769
Fixed income securities — Available for sale
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 1,526.4	$ 1,918.4
0-12 months, Gross Unrealized Losses	(19.1)	(11.9)
Over 12 months, Fair Market Value	2,121.1	984.3
Over 12 months, Gross Unrealized Losses	(58.8)	(21.9)
Total, Fair Market Value	3,647.5	2,902.7
Total, Gross Unrealized Losses	$ (77.9)	$ (33.8)
Number of Securities | Security	1,121	761
Short-term investments
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 34.5	$ 46.9
0-12 months, Gross Unrealized Losses	0.0	(0.1)
Over 12 months, Fair Market Value	0.0	0.0
Over 12 months, Gross Unrealized Losses	0.0	0.0
Total, Fair Market Value	34.5	46.9
Total, Gross Unrealized Losses	$ 0.0	$ (0.1)
Number of Securities | Security	12	8
U.S. government
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 180.2	$ 652.1
0-12 months, Gross Unrealized Losses	(0.7)	(5.1)
Over 12 months, Fair Market Value	740.6	259.8
Over 12 months, Gross Unrealized Losses	(15.4)	(6.5)
Total, Fair Market Value	920.8	911.9
Total, Gross Unrealized Losses	$ (16.1)	$ (11.6)
Number of Securities | security	103	101
U.S. agency
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 13.5	$ 20.1
0-12 months, Gross Unrealized Losses	(0.2)	(0.2)
Over 12 months, Fair Market Value	18.4	6.1
Over 12 months, Gross Unrealized Losses	(0.2)	0.0
Total, Fair Market Value	31.9	26.2
Total, Gross Unrealized Losses	$ (0.4)	$ (0.2)
Number of Securities | security	12	10
Municipal
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 3.1	$ 28.5
0-12 months, Gross Unrealized Losses	(0.1)	(0.2)
Over 12 months, Fair Market Value	25.0	0.0
Over 12 months, Gross Unrealized Losses	(0.7)	0.0
Total, Fair Market Value	28.1	28.5
Total, Gross Unrealized Losses	$ (0.8)	$ (0.2)
Number of Securities | security	9	9
Corporate
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 999.1	$ 699.3
0-12 months, Gross Unrealized Losses	(15.2)	(3.4)
Over 12 months, Fair Market Value	762.2	360.7
Over 12 months, Gross Unrealized Losses	(25.3)	(8.4)
Total, Fair Market Value	1,761.3	1,060.0
Total, Gross Unrealized Losses	$ (40.5)	$ (11.8)
Number of Securities | security	667	412
Non-U.S. government-backed corporate
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 14.5	$ 43.5
0-12 months, Gross Unrealized Losses	0.0	(0.3)
Over 12 months, Fair Market Value	25.8	13.3
Over 12 months, Gross Unrealized Losses	(0.2)	(0.2)
Total, Fair Market Value	40.3	56.8
Total, Gross Unrealized Losses	$ (0.2)	$ (0.5)
Number of Securities | security	12	15
Non-U.S government
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 64.0	$ 206.2
0-12 months, Gross Unrealized Losses	(0.3)	(0.8)
Over 12 months, Fair Market Value	91.0	32.0
Over 12 months, Gross Unrealized Losses	(0.5)	(0.4)
Total, Fair Market Value	155.0	238.2
Total, Gross Unrealized Losses	$ (0.8)	$ (1.2)
Number of Securities | security	57	47
Asset-backed
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 6.3	$ 11.1
0-12 months, Gross Unrealized Losses	0.0	0.0
Over 12 months, Fair Market Value	10.8	10.5
Over 12 months, Gross Unrealized Losses	(0.1)	(0.1)
Total, Fair Market Value	17.1	21.6
Total, Gross Unrealized Losses	$ (0.1)	$ (0.1)
Number of Securities | security	8	11
Agency mortgage-backed
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 245.7	$ 257.6
0-12 months, Gross Unrealized Losses	(2.6)	(1.9)
Over 12 months, Fair Market Value	447.3	301.9
Over 12 months, Gross Unrealized Losses	(16.4)	(6.3)
Total, Fair Market Value	693.0	559.5
Total, Gross Unrealized Losses	$ (19.0)	$ (8.2)
Number of Securities | security	253	156

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