Common stock Common stock issuance (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Aggregate Authorized Issuance of Debt Securities	$ 1,000,000,000.0
Common stock, shares authorized	500,000,000	500,000,000
Common Stock, Shares, Issued	200,922,790	196,564,907
Proceeds from Issuance of Common Stock	$ 106,848,000	$ 0	$ 0
At-the-market offering
Class of Stock [Line Items]
Common stock, shares authorized	10,000,000.0
Common Stock, Shares, Issued	3,600,000
Proceeds from Issuance of Common Stock	$ 94,000,000.0
Sales Commissions Paid from Issuance of Common Stock through the ATM Program	$ 950,000
Common Stock, Authorized Shares Remaining	6,400,000
K-Plan
Class of Stock [Line Items]
Common Stock, Authorized Shares Remaining	7,300,000