Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2019		Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019		Jun. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance					$ 1,609.3	$ 1,825.6		$ 1,609.3
Net realized/unrealized gains (losses) included in:
Net earnings	[1]					(12.6)		1.7
Other comprehensive income (loss)						57.1		(25.4)
Purchases						162.2		636.2
Sales						(305.8)		(381.4)
Settlements						(82.8)
Transfers into Level 3		$ 1.3				16.0		64.3
Transfers out of Level 3		(13.9)		$ (151.9)		(14.9)		(153.5)
Ending balance		1,644.8		1,751.2		1,644.8		1,751.2
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance					1.9	5.4		1.9
Net realized/unrealized gains (losses) included in:
Net earnings	[1]					(1.6)
Other comprehensive income (loss)								0.2
Purchases						0.3
Sales								(0.1)
Transfers into Level 3								4.3
Ending balance		4.1		6.3		4.1		6.3
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance					260.0	425.7		260.0
Net realized/unrealized gains (losses) included in:
Net earnings	[1]					(10.0)
Other comprehensive income (loss)						31.3		(6.9)
Purchases						90.9		117.3
Sales								(2.5)
Settlements						(3.6)
Transfers into Level 3						1.3		1.2
Transfers out of Level 3				(1.3)		(1.0)		(1.3)
Ending balance		534.6		367.8		534.6		367.8
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance					75.2	126.9		75.2
Net realized/unrealized gains (losses) included in:
Net earnings	[1]					(0.1)		(0.1)
Other comprehensive income (loss)						5.4		(2.2)
Purchases						13.9		23.8
Sales						(5.6)		(1.9)
Settlements						(7.7)
Transfers into Level 3						14.7
Transfers out of Level 3						(1.6)
Ending balance		145.9		94.8		145.9		94.8
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance					161.8			161.8
Net realized/unrealized gains (losses) included in:
Net earnings	[1]							(0.3)
Other comprehensive income (loss)								(5.3)
Sales								(5.6)
Transfers out of Level 3				(150.6)				(150.6)
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance					1.6			1.6
Net realized/unrealized gains (losses) included in:
Purchases						0.2
Transfers out of Level 3					(1.6)	(0.2)		(1.6)
Other Asset Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance					1,101.3	1,266.9		1,101.3
Net realized/unrealized gains (losses) included in:
Net earnings	[1]					(0.7)		1.0
Other comprehensive income (loss)						20.5		(7.7)
Purchases						56.8		495.1
Sales						(300.2)		(365.7)
Settlements						(71.5)
Transfers out of Level 3						(12.1)
Ending balance		959.7	[2]	1,224.0		959.7	[2]	1,224.0
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[3]				$ 7.5	0.7		7.5
Net realized/unrealized gains (losses) included in:
Net earnings	[1],[3]					(0.2)		1.1
Other comprehensive income (loss)	[3]					(0.1)		(3.5)
Purchases	[3]					0.1
Sales	[3]							(5.6)
Transfers into Level 3	[3]							58.8
Ending balance	[3]	$ 0.5		$ 58.3		$ 0.5		$ 58.3

[1] There were no other than temporary impairment (“OTTI”) losses recorded in net earnings related to Level 3 assets still held as of June 30, 2019 and 2018.

[2] Includes $952.5 million and $1,266.9 million of collateralized loan obligations as of June 30, 2019 and December 31, 2018, respectively.

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