Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Fair Value Measurements Significant Unobservable Inputs [Line Items]
Net derivative contract liabilities - Beginning Balance	$ (2,824)	$ (3,785)
Net derivative contract liabilities - Gains (losses) included in earnings	473	(737)
Net derivative contract liabilities - Gains (losses) included in other comprehensive income	(2)
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities	(2)	(11)
Net derivative contract liabilities - Acquisitions, dispositions and settlements	(50)	(35)
Net derivative contract liabilities - Transfers into/out of Level 3		195
Net derivative contract liabilities - Ending Balance	(2,405)	(4,373)
Fixed Maturities [Member]
Fair Value Measurements Significant Unobservable Inputs [Line Items]
Beginning Balance	64	100
Gains (losses) included in other comprehensive income	1	1
Acquisitions, dispositions and settlements	(58)	5
Transfers into/out of Level 3		(1)
Ending Balance	7	105
Equity Securities and Other Investments [Member]
Fair Value Measurements Significant Unobservable Inputs [Line Items]
Beginning Balance	17,257	21,403
Gains (losses) included in other comprehensive income	1,156	(927)
Net derivative contract liabilities - Transfers into/out of Level 3	(18,412)
Ending Balance	$ 1	$ 20,476