Capital Stock Dividends Declared (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Class of Stock [Line Items]
Total Amount	$ 21,925	$ 21,922	$ 21,816	$ 21,742	$ 43,847	$ 43,558
Regular Cash Dividend
Class of Stock [Line Items]
Dividends declared per common share (usd per share)	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.50	$ 0.50