Equity offerings (Details) - USD ($) $ in Thousands					12 Months Ended
	Aug. 09, 2016	Jul. 19, 2016	May 16, 2016	Mar. 05, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Proceeds from issuance of common stock, net of offering costs					$ 0	$ 276,052	$ 754,163
Common Stock
Class of Stock [Line Items]
Stock issued during the period (in shares)		13,000,000	10,925,000	69,000,000	0
Proceeds from issuance of common stock, net of offering costs		$ 136,300	$ 119,300	$ 754,200
Common Stock | Warburg Pincus LLC
Class of Stock [Line Items]
Stock issued during the period (in shares)				29,800,000
Common Stock | Over-Allotment Option
Class of Stock [Line Items]
Stock issued during the period (in shares)	1,950,000
Proceeds from issuance of common stock, net of offering costs	$ 20,500