Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				$ 1,041.4	$ 708.5
Net realized/unrealized gains (losses) included in:
Net earnings	[1]			2.9	3.6
Other comprehensive income				(6.1)	(4.6)
Purchases				255.0	781.1
Sales				(171.9)	(104.3)
Issuances				0.0	0.0
Settlements				(23.6)	(91.0)
Transfers into Level 3		$ 5.9		15.5	44.4
Transfers out of Level 3				(19.3)	(238.1)
Ending balance		1,093.9	$ 1,099.6	1,093.9	1,099.6
Common Stock
Net realized/unrealized gains (losses) included in:
Purchases					4.5
Issuances					0.0
Ending balance			4.5		4.5
Debt Securities
Net realized/unrealized gains (losses) included in:
Transfers into Level 3			6.3		44.4
Transfers out of Level 3		(3.0)		(19.3)
Debt Securities | U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				36.7	27.5
Net realized/unrealized gains (losses) included in:
Net earnings	[1]			0.5	(0.9)
Other comprehensive income				(0.8)	(0.3)
Purchases				33.9	21.6
Sales				(1.8)	(9.0)
Issuances				0.0	0.0
Settlements				(12.9)	(7.6)
Transfers into Level 3				9.8	15.0
Transfers out of Level 3				(13.8)
Ending balance		51.6	46.3	51.6	46.3
Debt Securities | Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				6.0	1.0
Net realized/unrealized gains (losses) included in:
Other comprehensive income				0.8
Purchases					2.5
Sales				(2.0)	(1.2)
Issuances				0.0	0.0
Settlements					(1.5)
Transfers into Level 3				0.7	0.3
Transfers out of Level 3				(5.5)
Ending balance			1.1		1.1
Debt Securities | Mortgage and asset backed securities | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				18.2	78.8
Net realized/unrealized gains (losses) included in:
Net earnings	[1]			0.5	4.1
Other comprehensive income				(0.6)	2.5
Sales					(12.2)
Issuances				0.0	0.0
Settlements				(2.2)	(8.3)
Ending balance		15.9	64.9	15.9	64.9
Debt Securities | Mortgage and asset backed securities | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				23.3	60.8
Net realized/unrealized gains (losses) included in:
Net earnings	[1]			(0.3)	(0.3)
Other comprehensive income				(1.5)	(1.3)
Purchases					22.0
Sales					(3.0)
Issuances				0.0	0.0
Settlements				(0.6)	(41.4)
Ending balance		20.9	36.8	20.9	36.8
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				933.1	258.4
Net realized/unrealized gains (losses) included in:
Net earnings	[1]			2.0	0.5
Other comprehensive income				(4.8)	(6.9)
Purchases				219.5	730.5
Sales				(167.9)	(78.7)
Issuances				0.0	0.0
Settlements				(7.9)	(11.2)
Transfers into Level 3					29.1
Ending balance		974.0	921.7	974.0	921.7
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[2]			24.1	282.0
Net realized/unrealized gains (losses) included in:
Net earnings	[1],[2]			0.2	0.2
Other comprehensive income	[2]			0.8	1.4
Purchases	[2]			1.6
Sales	[2]			(0.2)	(0.2)
Issuances	[2]			0.0	0.0
Settlements	[2]				(21.0)
Transfers into Level 3	[2]			5.0
Transfers out of Level 3	[2]				(238.1)
Ending balance	[2]	$ 31.5	$ 24.3	$ 31.5	$ 24.3

[1] There were no other than temporary impairment ("OTTI") losses recorded in net earnings related to Level 3 investments still held as of September 30, 2015 and 2014.

[2] Includes partnership and non-marketable equity investments accounted for on an AFS basis.