Long-term debt (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) Institution
Line of Credit Facility [Line Items]
Current borrowing capacity	$ 200.0	$ 200.0
Loss on extinguishment of debt	0.4	0.4
Letter of credit, outstanding amount	1.0	$ 1.0
Eurocurrency [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		1.00%
Eurocurrency [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		1.50%
Eurocurrency [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		2.50%
Base Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		0.50%
Base Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		1.50%
Federal Funds Rate [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		0.50%
Credit Agreement [Member]
Line of Credit Facility [Line Items]
Number of lending institutions | Institution		4
Maturity date		Sep. 29, 2022
Borrowing capacity with accordion feature	100.0	$ 100.0
Maximum borrowing capacity	300.0	$ 300.0
Debt covenant, consolidated debt service coverage ratio, minimum		2.50
Debt covenant, leverage ratio, maximum		3.50
Credit Agreement [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Commitment fee percentage		0.25%
Credit Agreement [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Commitment fee percentage		0.40%
Debt covenant, leverage ratio, maximum		4.00
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Outstanding credit facility	$ 0.0	$ 0.0