Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		$ 1,640.4	$ 1,825.6
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(13.8)	(15.3)
Other comprehensive income (loss)		35.5	63.4
Purchases		285.7	318.0
Sales		(64.5)	(384.0)
Issuances		0.0	0.0
Settlements		(128.9)	(163.7)
Transfers into Level 3		0.0	21.8
Transfers out of Level 3		(20.7)	(25.4)
Ending balance		1,733.7	1,640.4
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		2.0	5.4
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(0.7)	(1.7)
Other comprehensive income (loss)		0.0	0.0
Purchases		0.0	0.3
Sales		0.0	0.0
Issuances		0.0	0.0
Settlements		0.0	0.0
Transfers into Level 3		0.0	0.0
Transfers out of Level 3		0.0	(2.0)
Ending balance		1.3	2.0
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		605.0	425.7
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(3.8)	(6.5)
Other comprehensive income (loss)		22.7	39.4
Purchases		56.4	184.8
Sales		(2.0)	0.0
Issuances		0.0	0.0
Settlements		(46.7)	(37.0)
Transfers into Level 3		0.0	1.3
Transfers out of Level 3		0.0	(2.7)
Ending balance		631.6	605.0
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		168.7	126.9
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	0.1	(5.9)
Other comprehensive income (loss)		7.3	7.5
Purchases		32.0	42.7
Sales		(0.8)	(5.6)
Issuances		0.0	0.0
Settlements		(11.6)	(10.0)
Transfers into Level 3		0.0	14.7
Transfers out of Level 3		(6.2)	(1.6)
Ending balance		189.5	168.7
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1.9	0.0
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	0.0	0.0
Other comprehensive income (loss)		0.1	0.1
Purchases		0.9	1.8
Sales		0.0	0.0
Issuances		0.0	0.0
Settlements		(0.4)	0.0
Transfers into Level 3		0.0	0.0
Transfers out of Level 3		0.0	0.0
Ending balance		2.5	1.9
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		5.8	0.0
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	0.0	0.0
Other comprehensive income (loss)		0.1	0.0
Purchases		0.0	6.0
Sales		0.0	0.0
Issuances		0.0	0.0
Settlements		(0.1)	0.0
Transfers into Level 3		0.0	0.0
Transfers out of Level 3		0.0	(0.2)
Ending balance		5.8	5.8
Other asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		856.7	1,266.9
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(9.4)	(1.1)
Other comprehensive income (loss)		5.3	16.7
Purchases		196.4	82.3
Sales		(61.7)	(378.3)
Issuances		0.0	0.0
Settlements		(70.1)	(116.7)
Transfers into Level 3		0.0	5.8
Transfers out of Level 3		(14.5)	(18.9)
Ending balance		902.7	856.7
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[2]	0.3	0.7
Net realized/unrealized gains (losses) included in:
Net earnings	[1],[2]	0.0	(0.1)
Other comprehensive income (loss)	[2]	0.0	(0.3)
Purchases	[2]	0.0	0.1
Sales	[2]	0.0	(0.1)
Issuances	[2]	0.0	0.0
Settlements	[2]	0.0	0.0
Transfers into Level 3	[2]	0.0	0.0
Transfers out of Level 3	[2]	0.0	0.0
Ending balance	[2]	$ 0.3	$ 0.3

[1] There were no credit losses recorded in net earnings related to Level 3 instruments still held as of December 31, 2020 and 2019.

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