Employee compensation - Restricted Stock Option Awards - percentage of options exercisable (Details) - Stock Option Awards	6 Months Ended
	Jun. 30, 2017
Less than one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental percentage of option exercisable	0.00%
Cumulative percentage of option exercisable	0.00%
One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental percentage of option exercisable	25.00%
Cumulative percentage of option exercisable	25.00%
Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental percentage of option exercisable	25.00%
Cumulative percentage of option exercisable	50.00%
Three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental percentage of option exercisable	25.00%
Cumulative percentage of option exercisable	75.00%
Four
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental percentage of option exercisable	25.00%
Cumulative percentage of option exercisable	100.00%