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Computer Software
12 Months Ended
Dec. 31, 2017
Research and Development [Abstract]  
Computer Software
Computer Software

Computer software as of December 31, 2017 and 2016 consists of the following (in millions):
 
2017
 
2016
Software from business acquisitions
$
1,130

 
$
1,138

Capitalized software development costs
1,422

 
1,066

Purchased software
310

 
172

Computer software
2,862

 
2,376

Accumulated amortization
(1,134
)
 
(768
)
Computer software, net of accumulated amortization
$
1,728

 
$
1,608



Amortization expense for computer software was $436 million, $396 million and $229 million for the years ended December 31, 2017, 2016 and 2015, respectively.