Common stock offering (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended
	Sep. 07, 2017	Sep. 30, 2017
Common stock offering
Net proceeds from sale of stock		$ 649,387
Public Offering
Common stock offering
Issuance of shares of Common Stock	4,945,000
Per share price	$ 131.46
Net proceeds from sale of stock	$ 649,400
Maximum | Over-Allotment Option
Common stock offering
Underwriter purchase option period	30 days
Shares available for purchase by underwriter (in shares)	741,750