Issuances of Common Stock - Issuance of Common Stock in an Overnight Offering (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2017	May 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Issuance of Common Stock
Net proceeds from issuance of common shares			$ 0	$ 704,938	$ 383,572
Overnight Offering | Common stock
Issuance of Common Stock
Shares issued (in shares)	11,850,000	6,500,000	0
Underwriting discounts and offering costs	$ 29,800	$ 12,100
Net proceeds from issuance of common shares	$ 704,900	$ 383,600