Share-based compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jul. 15, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized share compensation expense	$ 4.6	$ 13.1
Unamortized share compensation expense, period of recognition	1 year 4 months 24 days	1 year 4 months
Fair value assumptions
Aggregate intrinsic value of options outstanding	$ 8.0	$ 19.6
Aggregate intrinsic value of options exercisable	7.1	13.4
Proceeds from stock options exercised	$ 5.1	$ 4.2
Non-Employee Directors
Fair value assumptions
Restricted stock awards issued	47,712	46,691	40,070
Employees
Fair value assumptions
Restricted stock awards issued	0	71,429	9,614
Management and director options
Fair value assumptions
Estimated share price (in dollars per share)			$ 15.05
Volatility assumption			23.10%
Risk free interest rate			2.20%
Expected life			6 years 6 months
Dividend yield			0.00%
Remaining contractual life	4 years 10 months 7 days	5 years 11 months 26 days
Management and director options | Management
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service condition met on each of first five anniversary dates following grant date	20.00%
Fair value assumptions
Award vesting period	5 years
Restricted share awards
Fair value assumptions
Award vesting period	3 years
Omnibus plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)				21,627,906
Number of shares available for future issuance (in shares)	9,418,538	9,786,902