Common stock Common stock issuance (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Shares, Issued	201,061,198	200,922,790
Proceeds from Issuance of Common Stock	$ 3,385,000	$ 106,848,000	$ 0
At-the-market offering
Class of Stock [Line Items]
Common Stock, Shares Authorized	6,400,000
Common Stock, Shares, Issued	0	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	7,200,000