Equity - Dividends on Common Stock (Details) - $ / shares	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Common dividend (usd per share)	$ 0.185	$ 0.185	$ 0.185	$ 0.185	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.16	$ 0.16	$ 0.16	$ 0.16	$ 0.74	$ 0.68	$ 0.64
Dividend, payable date	Jan. 16, 2018	Oct. 13, 2017	Jul. 14, 2017	Apr. 14, 2017	Jan. 13, 2017	Oct. 14, 2016	Jul. 15, 2016	Apr. 15, 2016		Oct. 15, 2015	Jul. 15, 2015	Apr. 15, 2015			Jan. 15, 2016
Dividend, date of record	Dec. 29, 2017	Sep. 29, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016		Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015			Dec. 31, 2015