Shareholders' Equity - Dividends (Details) - $ / shares		3 Months Ended								12 Months Ended
	Feb. 21, 2018	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Dividends (in dollars per share)		$ 0.1425	$ 0.1425	$ 0.1425	$ 0.1425	$ 0.13	$ 0.13	$ 0.13	$ 0.13	$ 0.57	$ 0.52	$ 0.48
Subsequent Event [Member]
Class of Stock [Line Items]
Dividends (in dollars per share)	$ 0.16
Increase in common stock dividend (as a percent)	12.00%