Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		$ 1,609.3	$ 1,019.7
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	1.6	3.9
Other comprehensive income		(11.4)	10.2
Purchases		164.4	429.9
Sales		(153.5)	(22.2)
Issuances		0.0	0.0
Settlements			(86.0)
Transfers into Level 3		63.0	2.6
Transfers out of Level 3		(1.6)	(4.1)
Ending balance		1,671.8	1,354.0
Equity Securities | Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance			4.3
Net realized/unrealized gains (losses) included in:
Other comprehensive income			0.2
Sales			(1.8)
Issuances			0.0
Transfers into Level 3			0.4
Transfers out of Level 3			(2.3)
Ending balance			0.8
Equity Securities | Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1.9
Net realized/unrealized gains (losses) included in:
Other comprehensive income		0.2
Purchases			2.4
Issuances		0.0	0.0
Transfers into Level 3		3.1
Ending balance		5.2	2.4
Debt Securities | U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		260.0	72.9
Net realized/unrealized gains (losses) included in:
Net earnings	[1]		(0.1)
Other comprehensive income		(5.4)	0.2
Purchases		36.2	43.8
Sales		(1.8)	(2.0)
Issuances		0.0	0.0
Settlements			(3.2)
Transfers into Level 3		1.2	1.9
Transfers out of Level 3			(1.8)
Ending balance		290.2	111.7
Debt Securities | Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		75.2	0.4
Net realized/unrealized gains (losses) included in:
Other comprehensive income		(1.3)
Purchases			3.1
Sales		(0.9)
Issuances		0.0	0.0
Transfers into Level 3			0.3
Ending balance		73.0	3.8
Debt Securities | Mortgage and asset backed securities | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		161.8	5.9
Net realized/unrealized gains (losses) included in:
Net earnings	[1]		0.1
Issuances		0.0	0.0
Settlements			(0.4)
Ending balance		161.8	5.6
Debt Securities | Mortgage and asset backed securities | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1.6	4.3
Net realized/unrealized gains (losses) included in:
Issuances		0.0	0.0
Settlements			(0.2)
Transfers out of Level 3		(1.6)
Ending balance			4.1
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		1,101.3	903.8
Net realized/unrealized gains (losses) included in:
Net earnings	[1]	0.4	1.5
Other comprehensive income		(3.3)	11.6
Purchases		128.2	380.6
Sales		(145.3)	(16.0)
Issuances		0.0	0.0
Settlements			(82.2)
Ending balance		1,081.3	1,199.3
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[2]	7.5	28.1
Net realized/unrealized gains (losses) included in:
Net earnings	[1],[2]	1.2	2.4
Other comprehensive income	[2]	(1.6)	(1.8)
Sales	[2]	(5.5)	(2.4)
Issuances	[2]	0.0	0.0
Transfers into Level 3	[2]	58.7
Ending balance	[2]	$ 60.3	$ 26.3

[1] There were no other than temporary impairment ("OTTI") losses recorded in net earnings related to Level 3 assets still held as of March 31, 2018 and 2017.

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