Share capital (Preferred stock) (Details) $ / shares in Units, $ in Thousands	Jun. 12, 2017 USD ($) $ / shares shares	Jun. 13, 2016 USD ($) $ / shares shares	Mar. 31, 2018 shares	Dec. 31, 2017 shares
Preferred shares - Shareholders' Equity
Preferred shares, shares issued			16,000	16,000
Preferred shares, shares outstanding			16,000	16,000
Series A preferred shares
Preferred shares - Shareholders' Equity
Preferred shares, dividend rate, percentage		5.875%
Preferred shares, shares issued		6,000	6,000	6,000
Preferred shares, shares outstanding			6,000	6,000
Preferred stock, par or stated value per share | $ / shares		$ 0.175
Preferred stock, liquidation preference, per share | $ / shares		$ 25,000
Net proceeds on issuance of preferred shares | $		$ 144,852
Series A depositary share equivalent
Preferred shares - Shareholders' Equity
Preferred shares equivalent depositary shares number		6,000,000
Proportionate interest of preferred shares, per depositary share		0.001
Preferred stock, liquidation preference, per share | $ / shares		$ 25
Series B preferred shares
Preferred shares - Shareholders' Equity
Preferred shares, dividend rate, percentage	5.80%
Preferred shares, shares issued	10,000		10,000	10,000
Preferred shares, shares outstanding			10,000	10,000
Preferred stock, par or stated value per share | $ / shares	$ 0.175
Preferred stock, liquidation preference, per share | $ / shares	$ 25,000
Net proceeds on issuance of preferred shares | $	$ 241,686
Series B depositary share equivalent
Preferred shares - Shareholders' Equity
Preferred shares equivalent depositary shares number	10,000,000
Proportionate interest of preferred shares, per depositary share	0.001
Preferred stock, liquidation preference, per share | $ / shares	$ 25