Issuances of Common Stock - At-the-Market (ATM) Programs (Details) - USD ($) $ in Thousands	12 Months Ended			39 Months Ended	40 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2018	Dec. 31, 2018
At-the-Market (ATM) Programs
Proceeds from At-the-Market (ATM) program	$ 1,125,364	$ 621,697	$ 166,781
ATM Programs
At-the-Market (ATM) Programs
Share issuances, net of costs (in shares)	19,138,610	10,914,088			33,546,139
Proceeds from At-the-Market (ATM) program	$ 1,100,000	$ 621,700			$ 2,000,000
Shares remaining for future issuance (in shares)	20,453,861				20,453,861
Prior ATM Program
At-the-Market (ATM) Programs
Share issuances, net of costs (in shares)				25,038,145
Current ATM Program | Maximum
At-the-Market (ATM) Programs
Shares authorized (in shares)				28,961,855