Available-for-sale securities - Summary of available-for-sale debt securities in a continuous unrealized loss position deemed not to be other-than-temporarily impaired (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ (22)	$ (114)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	11,149	17,583
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(80)	(1)
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	9,168	202
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(102)	(115)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20,317	17,785
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(14)	(60)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	7,674	8,685
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(32)	(1)
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	3,561	155
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(46)	(61)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	11,235	8,840
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)	(30)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	821	5,081
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(27)	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	3,240	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(29)	(30)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,061	5,081
Residential mortgage-backed and asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4)	(13)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	2,245	2,180
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(11)	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	1,206	42
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(15)	(13)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	3,451	2,222
U.S. government agencies securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(6)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	206	897
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(7)	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	700	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(8)	(6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	906	897
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(5)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	203	714
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3)	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	461	5
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4)	(5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 664	719
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value		15
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		15
Municipal debt securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value		11
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		$ 11