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Employee Benefit Plans - Employee Stock Purchase Plan and Valuation Assumptions (Details) - $ / shares
shares in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Employee Stock Purchase Plan      
Estimates of Fair Value      
Volatility 31.00% 27.00% 29.00%
Risk-free interest rate 0.80% 2.10% 1.90%
Expected life (years) 1 year 3 months 18 days 1 year 2 months 12 days 1 year 2 months 12 days
Dividend yield 3.30% 2.90% 2.70%
Weighted-average fair value per share (in dollars per share) $ 6.34 $ 6.65 $ 6.93
Market-based RSUs      
Estimates of Fair Value      
Volatility 25.00% 25.00% 28.00%
Risk-free interest rate 1.30% 2.40% 2.40%
Dividend yield 3.30% 2.80% 2.60%
Weighted-average fair value per share (in dollars per share) $ 26.32 $ 27.32 $ 28.39
Employee Stock Purchase Plan 2008      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common shares purchased through ESPP (in shares) 2.7 2.4 2.5
Average exercise price of shares purchased through ESPP (in dollars per share) $ 19.59 $ 22.04 $ 22.31