Stock option activity (Details) - Stock Options $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding
Shares Outstanding at Beginning of Period | shares	517
Shares Exercised | shares	(393)
Shares Outstanding at End of Period | shares	124
Shares Exercisable | shares	124
Weighted Average Exercise Price of Shares Outstanding at Beginning of Period | $ / shares	$ 23.97
Weighted Average Exercise Price of Shares Exercised | $ / shares	24.00
Weighted Average Exercise Price of Shares Outstanding at End of Period | $ / shares	23.86
Weighted Average Exercise Price of Exercisable Shares | $ / shares	$ 23.86
Weighted Average Remaining Contractual Term of Outstanding Options	2 years
Weighted Average Remaining Contractual Term of Exercisable Options	2 years
Options Outstanding, Intrinsic Value | $	$ 3.8
Options, Exercisable, Intrinsic Value | $	$ 3.8