Equity - Dividends on Common Stock (Details) - $ / shares	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Dividends per share of common stock
Dividends declared per common share (in usd per share)	$ 0.205	$ 0.205	$ 0.205	$ 0.185	$ 0.615	$ 0.555	$ 0.74
Dividends payment date	Oct. 15, 2018	Jul. 13, 2018	Apr. 13, 2018