Capital Stock Quarterly Dividend (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Common stock dividends	$ 21,933	$ 21,929	$ 21,925	$ 21,922	$ 21,837	$ 21,834	$ 21,816	$ 21,742	$ 87,709	$ 87,229
Regular Cash Dividend
Class of Stock [Line Items]
Dividends declared (usd per share)	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 1.0	$ 1.0	$ 1.00