Share-based compensation plans - Share Award and Incentive Plan, Restricted Share Awards (Details) - 2009 share award and incentive plan	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Restricted shares without performance criteria | Vesting January 1, 2021
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Restricted shares without performance criteria | Vesting January 1, 2022
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Restricted shares without performance criteria | Class A common shares | Vesting January 1, 2021
Share-based compensation plans
Granted (in shares) | shares	510
Restricted shares without performance criteria | Class A common shares | Vesting January 1, 2022
Share-based compensation plans
Granted (in shares) | shares	510
Granted December 20, 2018 | Restricted shares without performance criteria | Vesting December 20, 2019
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted December 20, 2018 | Restricted shares without performance criteria | Vesting December 20, 2020
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	0.01
Granted December 20, 2018 | Restricted shares without performance criteria | Vesting December 20, 2021
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	0.01
Granted December 20, 2018 | Restricted shares without performance criteria | Vesting December 20, 2022
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted December 20, 2018 | Restricted shares without performance criteria | Class A common shares
Share-based compensation plans
Granted (in shares) | shares	20,635
Granted December 20, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting December 20, 2019
Share-based compensation plans
Granted (in shares) | shares	20,635
Granted June 24, 2018 | Restricted shares without performance criteria | Vesting June 24, 2020
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted June 24, 2018 | Restricted shares without performance criteria | Vesting June 24, 2021
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	0.01
Granted June 24, 2018 | Restricted shares without performance criteria | Vesting June 24, 2022
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	0.01
Granted June 24, 2018 | Restricted shares without performance criteria | Vesting June 24, 2019
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	0.01
Granted June 24, 2018 | Restricted shares without performance criteria | On Retirement
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted June 24, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting June 24, 2020
Share-based compensation plans
Granted (in shares) | shares	2,850
Granted June 24, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting June 24, 2021
Share-based compensation plans
Granted (in shares) | shares	2,850
Granted June 24, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting June 24, 2022
Share-based compensation plans
Granted (in shares) | shares	2,850
Granted June 24, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting June 24, 2019
Share-based compensation plans
Granted (in shares) | shares	107,982
Granted June 24, 2018 | Restricted shares without performance criteria | Class A common shares | On Retirement
Share-based compensation plans
Granted (in shares) | shares	25,232
Granted March 24, 2018 | Restricted shares without performance criteria | Vesting March 24, 2019
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted March 24, 2018 | Restricted shares without performance criteria | Vesting March 24, 2020
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	0.01
Granted March 24, 2018 | Restricted shares without performance criteria | Vesting March 24, 2021
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	0.01
Granted March 24, 2018 | Restricted shares without performance criteria | Vesting March 24, 2022
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted March 24, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting March 24, 2019
Share-based compensation plans
Granted (in shares) | shares	59,100
Granted March 24, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting March 24, 2020
Share-based compensation plans
Granted (in shares) | shares	59,100
Granted March 24, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting March 24, 2021
Share-based compensation plans
Granted (in shares) | shares	59,100
Granted March 24, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting March 24, 2022
Share-based compensation plans
Granted (in shares) | shares	59,100
Granted March 24, 2018 | Restricted shares with performance criteria | Vesting March 24, 2021
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted March 24, 2018 | Restricted shares with performance criteria | Class A common shares | Vesting March 24, 2021
Share-based compensation plans
Granted (in shares) | shares	342,300
Granted January 15, 2018 | Restricted shares without performance criteria | Vesting January 15, 2021
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted January 15, 2018 | Restricted shares without performance criteria | Vesting January 15, 2022
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted January 15, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting January 15, 2021
Share-based compensation plans
Granted (in shares) | shares	7,750
Granted January 15, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting January 15, 2022
Share-based compensation plans
Granted (in shares) | shares	7,750
Granted January 1, 2018 | Restricted shares without performance criteria | Vesting January 1, 2019
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted January 1, 2018 | Restricted shares without performance criteria | Vesting January 1, 2020
Share-based compensation plans
Deferred Compensation Arrangement with Individual, Exercise Price (usd per share)	$ 0.01
Granted January 1, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting January 1, 2019
Share-based compensation plans
Granted (in shares) | shares	510
Granted January 1, 2018 | Restricted shares without performance criteria | Class A common shares | Vesting January 1, 2020
Share-based compensation plans
Granted (in shares) | shares	510