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Share-based compensation (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Payment Arrangement, Noncash Expense [Abstract]      
Total grant date fair value of stock awards vested $ 64,530 $ 41,854 $ 36,083
Total gross compensation recognized for stock awards 68,036 57,341 42,292
Capitalized stock compensation $ 24,396 $ 22,322 $ 16,682
Shares reserved for granting of future options and share awards 1,858,673    
Fair value assumptions, expected term 3 years 3 years  
Fair value assumptions, weighted average volatility rate (percent) 18.00% 16.00%  
Fair value assumptions, expected dividend rate (percent) 3.20% 3.10%  
Fair value assumptions, risk free interest rate (percent) 2.60% 2.15%  
Unrecognized compensation related to nonvested share awards $ 168,500    
Unrecognized compensation recognition period (in years) 4 years    
Weighted average period recognition period for unrecognized compensation 19 months    
Stock Award      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Outstanding, beginning balance (in shares) 1,712,376 1,394,582 1,135,788
Number of shares granted 768,625 741,244 688,295
Number of shares vested (666,836) (403,120) (423,705)
Number of shares forfeited (14,480) (20,330) (5,796)
Outstanding, ending balance (in shares) 1,799,685 1,712,376 1,394,582
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Weighed average grant date fair value per share, outstanding beginning balance $ 105.22 $ 95.79 $ 87.21
Weighed average grant date fair value per share, granted 134.70 121.20 108.22
Weighed average grant date fair value per share, vested 96.77 103.83 85.16
Weighed average grant date fair value per share, forfeited 119.88 106.38 101.45
Weighed average grant date fair value per share, outstanding ending balance $ 119.59 $ 105.22 $ 95.79