Equity - At-the-Market (ATM) Programs (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended		31 Months Ended
	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Oct. 31, 2017	Sep. 30, 2015
At-the-market (ATM) Programs
Proceeds from At-the-Market (ATM) program		$ 297,983	$ 52,442
Common stock
At-the-market (ATM) Programs
Share issuances, net of costs (in shares)	11,850,000
At market program | Maximum | Common stock
At-the-market (ATM) Programs
Shares authorized					17,000,000	12,000,000
Common stock | At market program
At-the-market (ATM) Programs
Share issuances, net of costs (in shares)		5,575,273	935,746	19,982,802
Proceeds from At-the-Market (ATM) program		$ 298,000	$ 52,400	$ 1,100,000