Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		$ 1,609.3	$ 1,019.7
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	1.5	12.3
Other comprehensive income		(68.9)	12.0
Purchases		1,089.5	1,421.2
Sales		(70.8)	(192.7)
Settlements		(578.4)	(590.9)
Transfers into Level 3		65.0	5.5
Transfers out of Level 3		(221.6)	(77.8)
Ending balance		1,825.6	1,609.3
Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance			4.3
Net realized/unrealized gains (losses) included in:
Net earnings	[1]		0.1
Sales			(2.6)
Transfers into Level 3			1.4
Transfers out of Level 3			(3.2)
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1.9
Net realized/unrealized gains (losses) included in:
Net earnings	[1]		(0.2)
Other comprehensive income		(2.7)	0.2
Purchases		2.0	5.6
Sales		(0.1)	(0.6)
Transfers into Level 3		4.3
Transfers out of Level 3			(3.1)
Ending balance		5.4	1.9
Foreign government obligations
Net realized/unrealized gains (losses) included in:
Purchases			4.8
Transfers out of Level 3			(4.8)
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		260.0	72.9
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(0.9)	(0.6)
Other comprehensive income		(11.5)	2.6
Purchases		182.3	252.3
Sales			(41.2)
Settlements		(4.8)	(8.5)
Transfers into Level 3		1.9	3.8
Transfers out of Level 3		(1.3)	(21.3)
Ending balance		425.7	260.0
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		75.2	0.4
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(0.1)	(1.5)
Other comprehensive income		(2.9)	0.4
Purchases		58.9	79.3
Sales			(0.6)
Settlements		(4.0)	(0.9)
Transfers into Level 3			0.3
Transfers out of Level 3		(0.2)	(2.2)
Ending balance		126.9	75.2
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		161.8	5.9
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(0.3)	0.3
Other comprehensive income		(5.3)	1.2
Purchases			161.1
Settlements		(5.6)	(1.4)
Transfers out of Level 3		(150.6)	(5.3)
Ending balance			161.8
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1.6	4.3
Net realized/unrealized gains (losses) included in:
Other comprehensive income			0.1
Purchases			11.2
Sales			(11.8)
Settlements			(0.5)
Transfers out of Level 3		(1.6)	(1.7)
Ending balance			1.6
Other Asset Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1,101.3	903.8
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	1.6	4.0
Other comprehensive income		(42.3)	16.8
Purchases		846.3	906.9
Sales		(65.0)	(114.4)
Settlements		(564.0)	(579.6)
Transfers out of Level 3		(11.0)	(36.2)
Ending balance		1,266.9	1,101.3
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[2]	7.5	28.1
Net realized/unrealized gains (losses) included in:
Net earnings	[1],[2]	1.2	10.2
Other comprehensive income	[2]	(4.2)	(9.3)
Sales	[2]	(5.7)	(21.5)
Transfers into Level 3	[2]	58.8
Transfers out of Level 3	[2]	(56.9)
Ending balance	[2]	$ 0.7	$ 7.5

[1] There were no OTTI losses recorded in net earnings related to Level 3 instruments still held as of December 31, 2018 and 2017.

[2] Includes partnership and non-marketable equity investments accounted for at fair value.