Stock-based compensation - Employee stock purchase plan (Details) (TI employees 2005 stock purchase plan, USD $)	12 Months Ended
	Dec. 31, 2014
TI employees 2005 stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, Beginning balance (in shares)	485,408
Granted (in shares)	1,673,479
Exercised (in shares)	(1,784,184)
Options outstanding, Ending balance (in shares)	374,703
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average exercise price, Outstanding, Beginning of period (in dollars per share)	$ 36.64
Weighted-average exercise price, Granted (in dollars per share)	$ 41.60
Weighted-average exercise price, Exercised (in dollars per share)	$ 39.44
Weighted-average exercise price, Outstanding, End of period (in dollars per share)	$ 45.46