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Employee compensation - Performance unit awards outstanding (Details) - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Performance unit awards          
Restricted stock awards          
Outstanding at the beginning of the period (in shares) 874,000 272,000 874,000 272,000 0
Granted (in shares)     1,801,000 602,000 272,000
Forfeited (in shares)     (350,000) 0 0
Vested (in shares)     0 0 0
Outstanding at the end of the period (in shares)     2,325,000 874,000 272,000
Performance Unit Awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Cash paid for performance units (in dollars per share) $ 143.75 $ 100.00      
2016 Performance Share Awards | May 25 and April 1 2016 awards | Performance unit awards          
Restricted stock awards          
Outstanding at the end of the period (in shares)     1,670,577