Issuances of Common Stock - At-the-Market (ATM) Programs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		46 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019
At-the-Market (ATM) Programs
Proceeds from At-the-Market (ATM) program			$ 122,155	$ 297,983
Current ATM Program
At-the-Market (ATM) Programs
Shares remaining for future issuance (in shares)	18,747,166		18,747,166		18,747,166
Common stock | ATM Programs
At-the-Market (ATM) Programs
Share issuances (in shares)	1,706,695	5,575,273	1,706,695	5,575,273	35,252,834
Proceeds from At-the-Market (ATM) program	$ 124,200	$ 298,000	$ 124,200	$ 298,000	$ 2,100,000
Common stock | Current ATM Program | Maximum
At-the-Market (ATM) Programs
Shares authorized (in shares)	28,961,855		28,961,855		28,961,855