Capital Stock (Share repurchase activity) (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
Equity [Abstract]
Number of Shares (in shares)	1,000,881	870,538	960,105	407,091	1,058,381	2,208,133	710,701	0	3,238,615	3,977,215
Average Repurchase Price per Share (in dollars per share)	$ 88.57	$ 87.59	$ 81.54	$ 94.79	$ 79.73	$ 75.49	$ 71.82	$ 0.00	$ 87.00	$ 75.96
Amount (in thousands)	$ 88,651	$ 76,248	$ 78,287	$ 38,588	$ 84,381	$ 166,685	$ 51,043	$ 0	$ 281,774	$ 302,109