Equity - At-the-Market (ATM) Programs (Details) - USD ($) $ in Thousands	1 Months Ended				12 Months Ended			28 Months Ended
	Mar. 31, 2017	May 31, 2016	Oct. 31, 2015	Apr. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Oct. 31, 2017	Sep. 30, 2015
At-the-market (ATM) Programs
Procceds from At-the-Market (ATM) program					$ 621,697	$ 166,781	$ 36,348
Common stock
At-the-market (ATM) Programs
Share issuances, net of costs (in shares)	11,850,000	6,500,000	11,500,000	5,500,000
ATM | Common stock
At-the-market (ATM) Programs
Share issuances, net of costs (in shares)					10,914,088	2,779,140		14,407,529
Procceds from At-the-Market (ATM) program					$ 621,700	$ 166,800		$ 824,800
ATM | Maximum | Common stock
At-the-market (ATM) Programs
Shares authorized									17,000,000	12,000,000