Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				$ 1,640,400,000	$ 1,825,600,000
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]			(13,200,000)	(12,900,000)
Other comprehensive income (loss)				16,100,000	59,800,000
Purchases				105,800,000	220,900,000
Sales				(57,500,000)	(384,400,000)
Issuances				0	0
Settlements				(81,100,000)	(115,000,000.0)
Transfers into Level 3			$ 5,800,000	0	21,800,000
Transfers out of Level 3		$ 0	(1,500,000)	(17,800,000)	(16,400,000)
Ending balance		1,592,700,000	1,599,400,000	1,592,700,000	1,599,400,000
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				2,000,000.0	5,400,000
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]			0	(1,600,000)
Other comprehensive income (loss)				0	0
Purchases				0	300,000
Sales				0	0
Issuances				0	0
Settlements				0	0
Transfers into Level 3				0	0
Transfers out of Level 3				0	0
Ending balance		2,000,000.0	4,100,000	2,000,000.0	4,100,000
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				605,000,000.0	425,700,000
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]			(3,800,000)	(9,900,000)
Other comprehensive income (loss)				19,200,000	40,000,000.0
Purchases				26,000,000.0	123,200,000
Sales				(2,000,000.0)	0
Issuances				0	0
Settlements				(14,700,000)	(6,300,000)
Transfers into Level 3				0	1,300,000
Transfers out of Level 3				0	(2,700,000)
Ending balance		629,700,000	571,300,000	629,700,000	571,300,000
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				168,700,000	126,900,000
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]			100,000	(200,000)
Other comprehensive income (loss)				5,100,000	3,500,000
Purchases				15,600,000	27,300,000
Sales				(800,000)	(5,600,000)
Issuances				0	0
Settlements				(10,400,000)	(8,900,000)
Transfers into Level 3				0	14,700,000
Transfers out of Level 3				(3,300,000)	(1,600,000)
Ending balance		175,000,000.0	156,100,000	175,000,000.0	156,100,000
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				1,900,000
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]			0
Other comprehensive income (loss)				(100,000)
Purchases				900,000
Sales				0
Issuances				0
Settlements				(200,000)
Transfers into Level 3				0
Transfers out of Level 3				0
Ending balance		2,500,000		2,500,000
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				5,800,000	0
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]			0	0
Other comprehensive income (loss)				100,000	0
Purchases				0	1,700,000
Sales				0	0
Issuances				0	0
Settlements				0	0
Transfers into Level 3				0	0
Transfers out of Level 3				0	0
Ending balance		5,900,000	1,700,000	5,900,000	1,700,000
Other Asset Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance				856,700,000	1,266,900,000
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]			(9,500,000)	(1,100,000)
Other comprehensive income (loss)				(8,200,000)	16,700,000
Purchases				63,300,000	68,300,000
Sales				(54,700,000)	(378,800,000)
Issuances				0	0
Settlements				(55,800,000)	(99,800,000)
Transfers into Level 3				0	5,800,000
Transfers out of Level 3				(14,500,000)	(12,100,000)
Ending balance		777,300,000	865,900,000	777,300,000	865,900,000
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[2]			300,000	700,000
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1],[2]			0	(100,000)
Other comprehensive income (loss)	[2]			0	(400,000)
Purchases	[2]			0	100,000
Sales	[2]			0	0
Issuances	[2]			0	0
Settlements	[2]			0	0
Transfers into Level 3	[2]			0	0
Transfers out of Level 3	[2]			0	0
Ending balance	[2]	$ 300,000	$ 300,000	$ 300,000	$ 300,000

[1] There were no credit losses recorded in net earnings related to Level 3 assets still held as of September 30, 2020 and 2019.

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