Debt (Principal Payments For Investment Agreements, MTNs and VIE Obligations) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
2022		$ 265.0
2023		0.0
Thereafter		1,248.0
Total		1,513.0
Accrued interest		751.0	$ 624.0
Investment Agreement Obligations [Member]
Debt Instrument [Line Items]
2019		7.0
2020		35.0
2021		2.0
2022		2.0
2023		20.0
Thereafter		295.0
Total expected principal payments	[1]	361.0
Less discount and other adjustments	[2]	50.0
Total		311.0
Accrued interest		5.0
Carrying amount adjustment		$ 4.0
Latest maturity year		2037
Medium Term Notes [Member]
Debt Instrument [Line Items]
2019		$ 58.0
2020		0.0
2021		0.0
2022		58.0
2023		0.0
Thereafter		835.0
Total expected principal payments	[3]	951.0
Less discount and other adjustments	[4]	229.0
Total		722.0
Accrued interest		5.0
Carrying amount adjustment		$ 34.0
Latest maturity year		2036
Variable Interest Entity Debt [Member] | International And Structured Finance Insurance [Member]
Debt Instrument [Line Items]
2019		$ 108.0
2020		396.0
2021		72.0
2022		41.0
2023		13.0
Thereafter		1,111.0
Total expected principal payments	[5]	1,741.0
Net fair value		$ 480.0
Latest maturity year		2052

[1] Amounts reflect principal due at maturity for investment agreements issued at a discount.

[2] Discount is net of carrying amount adjustment of $4 million and accrued interest adjustment of $5 million.

[3] Amounts reflect principal due at maturity for notes issued at a discount.

[4] Discount is net of carrying amount adjustment of $34 million and accrued interest adjustment of $5 million.

[5] Includes $480 million of VIE notes accounted for at fair value as of December 31, 2018.