Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		$ 1,067.8	$ 1,041.4
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	2.6	0.7
Other comprehensive income		(15.7)	8.1
Purchases		10.6	42.6
Sales		(28.0)	(53.8)
Issuances		0.0	0.0
Settlements		(1.9)	(5.7)
Transfers into Level 3		2.7
Transfers out of Level 3		(0.2)	(10.4)
Ending balance		1,037.9	1,022.9
Common Stock
Net realized/unrealized gains (losses) included in:
Issuances		0.0
Transfers into Level 3		1.0
Ending balance		1.0
Debt Securities | U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		49.8	36.7
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	0.1	0.1
Other comprehensive income		0.2	0.4
Purchases		0.3	6.7
Sales		(1.2)	(0.5)
Issuances		0.0	0.0
Settlements		(0.9)	(4.0)
Transfers into Level 3		1.7
Transfers out of Level 3			(6.8)
Ending balance		50.0	32.6
Debt Securities | Mortgage and asset backed securities | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		14.9	18.2
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	0.1	0.1
Other comprehensive income		(0.2)
Issuances		0.0	0.0
Settlements		(0.5)	(0.7)
Ending balance		14.3	17.6
Debt Securities | Mortgage and asset backed securities | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		20.2	23.3
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(0.1)	(0.1)
Other comprehensive income		(0.1)	(0.4)
Purchases			0.9
Issuances		0.0	0.0
Settlements		(0.2)	(0.5)
Ending balance		19.8	23.2
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		953.0	933.1
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	1.0	0.6
Other comprehensive income		(14.3)	7.8
Purchases		10.3	35.0
Sales		(25.3)	(53.1)
Issuances		0.0	0.0
Settlements		(0.3)	(0.5)
Ending balance		924.4	922.9
Debt Securities | Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance			6.0
Net realized/unrealized gains (losses) included in:
Sales			(0.2)
Issuances			0.0
Transfers out of Level 3			(3.6)
Ending balance			2.2
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[2]	29.9	24.1
Net realized/unrealized gains (losses) included in:
Net earnings	[1],[2]	1.5
Other comprehensive income	[2]	(1.3)	0.3
Sales	[2]	(1.5)
Issuances	[2]	0.0	0.0
Transfers out of Level 3	[2]	(0.2)
Ending balance	[2]	$ 28.4	$ 24.4

[1] There were no other than temporary impairment ("OTTI") losses recorded in net earnings related to Level 3 investments still held as of March 31, 2016 and 2015.

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