Dividends - Quarterly Dividend Payments (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Dividends [Abstract]
Cash dividends paid per common share (in dollars per share)	$ 0.185	$ 0.18375	$ 0.1825	$ 0.18125
Total cash dividends paid	$ 13,493	$ 13,398	$ 13,293	$ 13,158	$ 53,342	$ 51,375	$ 49,077