Investments in fixed maturity securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Schedule of investments in fixed maturity securities

Investments in fixed maturity securities as of December 31, 2017 and 2016 are summarized by type below (in millions).

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2017
U.S. Treasury, U.S. government corporations and agencies	$	3,975	$	4	$	(26	)	$	3,953
States, municipalities and political subdivisions		847		19		(12	)		854
Foreign governments		8,572		274		(24	)		8,822
Corporate bonds		6,279		588		(5	)		6,862
Mortgage-backed securities		772		92		(2	)		862
	$	20,445	$	977	$	(69	)	$	21,353
December 31, 2016
U.S. Treasury, U.S. government corporations and agencies	$	4,519	$	16	$	(8	)	$	4,527
States, municipalities and political subdivisions		1,159		58		(1	)		1,216
Foreign governments		8,860		207		(66	)		9,001
Corporate bonds		6,899		714		(9	)		7,604
Mortgage-backed securities		967		123		(6	)		1,084
	$	22,404	$	1,118	$	(90	)	$	23,432

Schedule of amortized cost and estimated fair value of fixed maturity securities

The amortized cost and estimated fair value of fixed maturity securities at December 31, 2017 are summarized below by contractual maturity dates. Actual maturities may differ from contractual maturities due to early call or prepayment rights held by issuers. Amounts are in millions.

	Due in one year or less	Due after one year through five years	Due after five years through ten years	Due after ten years		Mortgage-backed securities	Total
Amortized cost	$6,123	$ 11,020	$  510	$	2,020	$   772	$	20,445
Fair value	6,175	11,183	557		2,576	862		21,353