Capital Stock and Shares Compensation Plans - Stock options transactions (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Outstanding stock options [Roll forward]
Beginning balance	2,223,000
Granted	435,000
Exercised	(285,938)	(1,007,676)	(990,383)
Forfeited	(1,000)
Ending balance	2,371,000	2,223,000
Vested and expected to vest	2,315,000
Options exercisable	1,660,000
Weighted-average exercise price [Abstract]
Beginning balance	$ 52.81
Ending balance	57.38	$ 52.81
Vested and expected to vest	57.06
Options exercisable	$ 50.87
Weighted-Average Remaining Term	4 years 6 months
Weight-Average remaining term vested and expected to vest	4 years 4 months 24 days
Weighted Average remaining term options exercisable	3 years 7 months 6 days
Aggregate intrinsic value, outstanding	$ 14,936
Aggregated intrinsic value, vested and expected to vest	14,936
Aggregate intrinsic value, exercisable	$ 14,936