Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 2,047.1	$ 2,080.0
Gross unrealized holding gains	122.0	56.2
Gross unrealized holding losses	(2.8)	(8.3)
Fair value	2,166.3	2,127.9
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	510.9	569.2
Gross unrealized holding gains	48.1	12.3
Gross unrealized holding losses	0.0	(3.3)
Fair value	559.0	578.2
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	527.5	404.3
Gross unrealized holding gains	26.7	21.1
Gross unrealized holding losses	(1.5)	0.0
Fair value	552.7	425.4
Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	451.7	460.5
Gross unrealized holding gains	21.9	11.7
Gross unrealized holding losses	0.0	(0.4)
Fair value	473.6	471.8
U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	557.0	646.0
Gross unrealized holding gains	25.3	11.1
Gross unrealized holding losses	(1.3)	(4.6)
Fair value	$ 581.0	$ 652.5