Issuances of Common Stock - At-the-Market (ATM) Programs (Details) - USD ($) $ in Thousands	9 Months Ended		37 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Oct. 31, 2017	Sep. 30, 2015
At-the-market (ATM) Programs
Proceeds from At-the-Market (ATM) program	$ 588,860	$ 487,998
Common stock | At The Market Program
At-the-market (ATM) Programs
Shares issued (in shares)	10,630,616	8,506,559	25,038,145
Proceeds from At-the-Market (ATM) program	$ 588,900	$ 488,000	$ 1,400,000
Common stock | At The Market Program | Maximum
At-the-market (ATM) Programs
Shares authorized (in shares)				17,000,000	12,000,000