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Mortgage servicing rights - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Mortgage servicing rights sold   $ 39,428 $ 11,686 $ 34,118
Mortgage loans serviced   3,181,483 $ 1,086,465 $ 3,370,395
Mortgage escrow deposit   $ 53,468    
Scenario, Forecast        
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Mortgage loans serviced related to bulk sale of mortgage servicing rights $ 2,061,175      
Subsequent Event        
Transfer of Financial Assets Accounted for as Sales [Line Items]        
Mortgage servicing rights sold 29,416      
Mortgage loans serviced related to bulk sale of mortgage servicing rights $ 2,034,374