Capital Stock (Share repurchase activity) (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
Equity [Abstract]
Number of Shares	1,058,381	2,208,133	710,701	0	3,977,215
Average Repurchase Price per Share (in dollars per share)	$ 79.73	$ 75.49	$ 71.82	$ 0.00	$ 75,960.00
Amount (in thousands)	$ 84,381	$ 166,685	$ 51,043	$ 0	$ 302,109