Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 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]	8.1	3.3
Other comprehensive income		27.7	(28.3)
Purchases		225.7	270.6
Sales		(113.6)	(186.9)
Issuances		0.0	0.0
Settlements		(186.4)	(29.3)
Transfers into Level 3		10.6	19.9
Transfers out of Level 3		(20.2)	(22.9)
Ending balance		1,019.7	1,067.8
Equity Securities | Common Stock
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	(0.6)
Other comprehensive income		1.8
Purchases		2.2
Sales		(0.2)
Issuances		0.0
Transfers into Level 3		1.4
Transfers out of Level 3		(0.3)
Ending balance		4.3
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]	(0.3)	(0.6)
Other comprehensive income		0.1	(1.3)
Purchases		46.1	35.5
Sales		(15.0)	(1.9)
Issuances		0.0	0.0
Settlements		(6.5)	(16.9)
Transfers into Level 3		7.3	14.2
Transfers out of Level 3		(8.6)	(15.9)
Ending balance		72.9	49.8
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:
Other comprehensive income			0.8
Purchases		0.4
Sales		(0.2)	(2.0)
Issuances		0.0	0.0
Transfers into Level 3		1.9	0.7
Transfers out of Level 3		(1.7)	(5.5)
Ending balance		0.4
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]	0.3	0.6
Other comprehensive income		(0.5)	(1.1)
Sales		(7.0)
Issuances		0.0	0.0
Settlements		(1.8)	(2.8)
Ending balance		5.9	14.9
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]	(0.2)	(0.4)
Other comprehensive income		0.4	(1.0)
Sales		(9.7)
Issuances		0.0	0.0
Settlements		(0.6)	(1.7)
Transfers out of Level 3		(5.8)
Ending balance		4.3	20.2
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]	4.2	2.7
Other comprehensive income		27.4	(25.9)
Purchases		177.0	233.3
Sales		(76.8)	(182.3)
Issuances		0.0	0.0
Settlements		(177.5)	(7.9)
Transfers out of Level 3		(3.5)
Ending balance		903.8	953.0
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]	4.7	1.0
Other comprehensive income	[2]	(1.5)	0.2
Purchases	[2]		1.8
Sales	[2]	(4.7)	(0.7)
Issuances	[2]	0.0	0.0
Transfers into Level 3	[2]		5.0
Transfers out of Level 3	[2]	(0.3)	(1.5)
Ending balance	[2]	$ 28.1	$ 29.9

[1] There were no OTTI losses recorded in net earnings related to Level 3 instruments still held as of December 31, 2016 and 2015.

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