Employee plans - Summary of Stock Option Activity (Details) - Employee Stock Option $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) $ / shares shares
Shares
Outstanding at beginning of period (in shares) | shares	2,723
Granted (in shares) | shares	11
Exercised (in shares) | shares	0
Forfeited and expired (in shares) | shares	(635)
Outstanding at end of period (in shares) | shares	2,099
Expected to vest at end of period (in shares) | shares	608
Exercisable at end of period (in shares) | shares	1,478
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 27.88
Granted (in dollars per share) | $ / shares	10.85
Exercised (in dollars per share) | $ / shares	0.00
Forfeited and expired (in dollars per share) | $ / shares	35.76
Outstanding at end of period (in dollars per share) | $ / shares	25.41
Expected to vest at end of period (in dollars per share) | $ / shares	26.06
Exercisable at end of period (in dollars per share) | $ / shares	$ 25.17
Weighted- Average Remaining Contractual Term (years)
Outstanding at end of period	2 years 3 months 11 days
Expected to vest at end of period	3 years 8 months 12 days
Exercisable at end of period	1 year 8 months 1 day
Aggregate Intrinsic Value
Outstanding at end of period | $	$ 0.0
Expected to vest at end of period | $	0.0
Exercisable at end of period | $	$ 0.0