Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	$ 2,051.8	$ 2,080.0
Gross unrealized holding gains	88.5	56.2
Gross unrealized holding losses	(12.0)	(8.3)
Fair value	2,128.3	2,127.9
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	559.3	569.2
Gross unrealized holding gains	38.0	12.3
Gross unrealized holding losses	(0.2)	(3.3)
Fair value	597.1	578.2
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	392.9	404.3
Gross unrealized holding gains	20.8	21.1
Gross unrealized holding losses	0.0	0.0
Fair value	413.7	425.4
Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	470.1	460.5
Gross unrealized holding gains	5.0	11.7
Gross unrealized holding losses	(7.2)	(0.4)
Fair value	467.9	471.8
U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	629.5	646.0
Gross unrealized holding gains	24.7	11.1
Gross unrealized holding losses	(4.6)	(4.6)
Fair value	$ 649.6	$ 652.5