Income taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
NOL and credit carryforwards	$ 812	$ 688
Accrued expenses	362	562
Expenses capitalized for tax	155	255
Stock-based compensation	99	167
Other	154	117
Total deferred income tax assets	1,582	1,789
Valuation allowance	(497)	(381)
Net deferred income tax assets	1,085	1,408
Deferred income tax liabilities:
Acquired intangible assets	(1,748)	(3,139)
Debt	(184)	(345)
Other	(240)	(307)
Total deferred income tax liabilities	(2,172)	(3,791)
Total deferred income taxes, net	$ (1,087)	$ (2,383)