Long-term debt (Details) $ / shares in Units, $ in Millions		1 Months Ended
	Aug. 12, 2016 shares $ / shares	Jan. 29, 2014 USD ($) shares $ / shares	Dec. 31, 2015 USD ($)
Amount of debt issuance costs in prepaid expenses and other current assets, netted against long term debt [Member]
Long-Term Debt [Line Items]
Debt issuance costs			$ 1.2
Amount of debt issuance costs in other assets, netted against long term debt [Member]
Long-Term Debt [Line Items]
Debt issuance costs			$ 4.9
Convertible Senior Notes Due 2021 [Member]
Long-Term Debt [Line Items]
Face amount of debt instrument		$ 250.0
Interest rate, stated percentage		2.875%
Maturity date		Jan. 15, 2021
Conversion rate of notes per $1,000 principal amount (in shares) | shares	32.3860	30.8821
Conversion price per share (in dollars per share) | $ / shares	$ 30.88	$ 32.38