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Capital Stock and Long-term Retention Plan - Weighted-average assumptions (Details) - Long-Term Retention Plan - EquityInstruments
EquityInstruments in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Assumptions:          
Dividend yield 0.38% 0.38% 0.33% 0.39% 0.54%
Expected volatility 24.58% 27.60% 26.92% 19.07% 24.00%
Risk-free interest rate 7.04% 4.83% 4.61% 4.68% 4.79%
Expected average life of awards 2 years 11 months 16 days 3 years 3 years 3 years 3 years
CPOs          
Arrangements:          
Number of CPOs or CPOs equivalent granted (in units) 37,000 39,000 39,000 39,000 39,000
Contractual life 3 years 3 years 3 years 3 years 3 years