Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost		$ 2,517.5
Gross unrealized holding gains		126.9
Gross unrealized holding losses		(30.3)
Fair value		2,614.1
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,199.0	2,173.1
Gross unrealized holding gains	22.8	40.0
Gross unrealized holding losses	(40.2)	(20.3)
Fair value	2,181.6	2,192.8
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	413.2	433.8
Gross unrealized holding gains	6.9	9.3
Gross unrealized holding losses	(9.2)	(6.2)
Fair value	410.9	436.9
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	497.6	507.1
Gross unrealized holding gains	10.7	19.1
Gross unrealized holding losses	(1.1)	(0.4)
Fair value	507.2	525.8
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	516.4	527.5
Gross unrealized holding gains	1.9	4.5
Gross unrealized holding losses	(8.5)	(2.3)
Fair value	509.8	529.7
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	771.8	704.7
Gross unrealized holding gains	3.3	7.1
Gross unrealized holding losses	(21.4)	(11.4)
Fair value	$ 753.7	700.4
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost		318.6
Gross unrealized holding gains		56.7
Gross unrealized holding losses		(10.0)
Fair value		365.3
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost		62.4
Gross unrealized holding gains		35.1
Gross unrealized holding losses		(0.7)
Fair value		96.8
Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost		256.2
Gross unrealized holding gains		21.6
Gross unrealized holding losses		(9.3)
Fair value		268.5
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost		25.8
Gross unrealized holding gains		30.2
Gross unrealized holding losses		0.0
Fair value		$ 56.0