Investments - Available for Sale Securities Continuous Unrealized Loss Position Fair Value and Aggregate Losses (Detail) $ in Millions	Mar. 31, 2020 USD ($) position	Dec. 31, 2019 USD ($) position
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 341.2	$ 262.6
Less than 12 months, Unrealized losses	$ (11.2)	$ (4.0)
Less than 12 months, Number of positions | position	72	32
12 months or more, Fair value	$ 28.1	$ 336.3
12 months or more, Unrealized losses	$ (0.8)	$ (4.3)
12 months or more, Number of positions | position	7	50
Total, Fair value	$ 369.3	$ 598.9
Total, Unrealized losses	$ (12.0)	$ (8.3)
Total, Number of positions | position	79	82
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 22.2	$ 136.0
Less than 12 months, Unrealized losses	$ (0.2)	$ (2.5)
Less than 12 months, Number of positions | position	7	17
12 months or more, Fair value	$ 0.0	$ 157.6
12 months or more, Unrealized losses	$ 0.0	$ (0.8)
12 months or more, Number of positions | position	0	11
Total, Fair value	$ 22.2	$ 293.6
Total, Unrealized losses	$ (0.2)	$ (3.3)
Total, Number of positions | position	7	28
Corporate Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 205.9	$ 0.0
Less than 12 months, Unrealized losses	$ (7.1)	$ 0.0
Less than 12 months, Number of positions | position	38	0
12 months or more, Fair value	$ 6.0	$ 40.8
12 months or more, Unrealized losses	$ (0.1)	$ (0.4)
12 months or more, Number of positions | position	2	7
Total, Fair value	$ 211.9	$ 40.8
Total, Unrealized losses	$ (7.2)	$ (0.4)
Total, Number of positions | position	40	7
U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 113.1	$ 126.6
Less than 12 months, Unrealized losses	$ (3.9)	$ (1.5)
Less than 12 months, Number of positions | position	27	15
12 months or more, Fair value	$ 22.1	$ 137.9
12 months or more, Unrealized losses	$ (0.7)	$ (3.1)
12 months or more, Number of positions | position	5	32
Total, Fair value	$ 135.2	$ 264.5
Total, Unrealized losses	$ (4.6)	$ (4.6)
Total, Number of positions | position	32	47