Subsequent events (Details) - USD ($) $ in Thousands	Apr. 12, 2019	Apr. 04, 2019	Mar. 31, 2019	Mar. 22, 2019	Jan. 18, 2019	Dec. 31, 2018	Mar. 31, 2018
Subsequent Event [Line Items]
Long-term debt			$ 2,198,027			$ 2,108,695
Short-term Debt			70,000			0	$ 0
Term Loan Agreements
Subsequent Event [Line Items]
Short-term Debt				$ 40,000	$ 30,000
Term Loan Agreements | Subsequent Event [Member]
Subsequent Event [Line Items]
Short-term Debt	$ 50,000
Senior Notes Member]
Subsequent Event [Line Items]
Long-term debt			$ 1,381,000			$ 1,381,000
Senior Notes Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Long-term debt		$ 150,000