Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		$ 1,640.4	$ 1,825.6
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]	0.1	(9.9)
Other comprehensive income (loss)		(128.8)	32.5
Purchases		58.9	69.4
Sales		(8.3)	(122.5)
Settlements		(37.2)	(39.4)
Transfers into Level 3			14.7
Transfers out of Level 3		(14.7)	(1.0)
Ending balance		1,510.4	1,769.4
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		2.0	5.4
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)			(0.8)
Ending balance		2.0	4.6
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		605.0	425.7
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]	0.2	(9.9)
Other comprehensive income (loss)		(18.8)	15.7
Purchases		6.7	5.8
Settlements		(3.6)	(2.6)
Transfers out of Level 3			(1.0)
Ending balance		589.5	433.7
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		168.7	126.9
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)		(7.3)	2.6
Purchases		7.7	13.8
Sales			(5.6)
Settlements		(1.1)	(1.0)
Transfers into Level 3			14.7
Transfers out of Level 3		(3.3)
Ending balance		164.7	151.4
RMBS
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 (loss)		(0.2)
Purchases		0.9
Settlements		(0.1)
Ending balance		2.5
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		5.8
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)		(1.3)
Purchases			0.2
Ending balance		4.5	0.2
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[2]	0.3	0.7
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)	[2]	0.2	(0.2)
Ending balance	[2]	0.5	0.5
Other Asset Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		856.7	1,266.9
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]	(0.1)
Other comprehensive income (loss)		(101.4)	15.2
Purchases		43.6	49.6
Sales		(8.3)	(116.9)
Settlements		(32.4)	(35.8)
Transfers out of Level 3		(11.4)
Ending balance		$ 746.7	$ 1,179.0

[1] There were no credit losses recorded in net earnings related to Level 3 assets still held as of March 31, 2020 and 2019.

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