Issuances of Common Stock - At-the-Market (ATM) Program (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
At-the-Market (ATM) Program
Gross proceeds			$ 689,641	$ 588,860
ATM Program
At-the-Market (ATM) Program
Shares remaining for future issuance (in shares)	11,083,783		11,083,783
Common stock | ATM Program
At-the-Market (ATM) Program
Shares of common stock issued (in shares)	7,663,383	5,055,343	9,370,078	10,630,616
Gross proceeds	$ 570,300	$ 290,900	$ 694,400	$ 588,900
Common stock | ATM Program | Maximum
At-the-Market (ATM) Program
Authorized common shares to be issued (in shares)	28,961,855		28,961,855