Dividends - Quarterly Dividend Payments (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends [Abstract]
Cash dividends paid per common share (in dollars per share)	$ 0.19000	$ 0.18875	$ 0.18750	$ 0.18625
Total cash dividends paid	$ 13,987	$ 13,883	$ 13,750	$ 13,624	$ 55,244	$ 53,342	$ 51,375