Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,444.5	$ 2,295.0
Gross unrealized holding gains	118.5	125.4
Gross unrealized holding losses	(25.8)	(24.0)
Fair value	2,537.2	2,396.4
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,095.9	1,972.9
Gross unrealized holding gains	35.8	46.4
Gross unrealized holding losses	(22.4)	(18.6)
Fair value	2,109.3	2,000.7
Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	323.1	265.2
Gross unrealized holding gains	63.1	50.7
Gross unrealized holding losses	(3.4)	(5.3)
Fair value	382.8	310.6
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	25.5	56.9
Gross unrealized holding gains	19.6	28.3
Gross unrealized holding losses	0.0	(0.1)
Fair value	45.1	85.1
U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	401.9	219.8
Gross unrealized holding gains	8.9	6.6
Gross unrealized holding losses	(6.1)	(2.0)
Fair value	404.7	224.4
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	634.6	804.0
Gross unrealized holding gains	12.3	22.5
Gross unrealized holding losses	(3.2)	(1.7)
Fair value	643.7	824.8
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	445.7	500.3
Gross unrealized holding gains	6.1	5.8
Gross unrealized holding losses	(2.2)	(11.7)
Fair value	449.6	494.4
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	613.7	448.8
Gross unrealized holding gains	8.5	11.5
Gross unrealized holding losses	(10.9)	(3.2)
Fair value	611.3	457.1
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	108.9	136.3
Gross unrealized holding gains	32.7	29.4
Gross unrealized holding losses	(2.6)	(4.0)
Fair value	139.0	161.7
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	57.2	53.3
Gross unrealized holding gains	21.9	16.5
Gross unrealized holding losses	0.0	(0.2)
Fair value	79.1	69.6
Mutual and exchange traded funds [Domain] [Member] | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	157.0	75.6
Gross unrealized holding gains	8.5	4.8
Gross unrealized holding losses	(0.8)	(1.1)
Fair value	$ 164.7	$ 79.3