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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Assumptions:          
Dividend yield 1.38% 0.82% 0.55% 0.38% 0.38%
Expected volatility 35.13% 30.47% 25.38% 24.58% 27.60%
Risk-free interest rate 5.74% 6.88% 7.17% 7.04% 4.83%
Expected average life of awards 3 years 2 years 8 months 1 day 3 years 2 years 11 months 16 days 3 years
CPOs          
Arrangements:          
Number of CPOs or CPOs equivalent granted (in units) 39,200 72,558 32,500 37,000 39,000
Contractual life 3 years 2 years 8 months 1 day 3 years 3 years 3 years