Share capital (Preferred stock) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Class of Stock [Line Items]
Preferred shares, shares issued at beginning of period	0
Preferred shares, shares outstanding at beginning of period	0
Preferred shares, shares issued at end of period	6,000
Preferred shares, shares outstanding at end of period	6,000
Preferred shares - Shareholders' Equity
Preferred shares equivalent depositary shares number	6,000,000
Proportionate interest of Series A Preferred shares, per depositary share	0.10%
Net proceeds on issuance of preferred shares	$ 144,852,000	$ 0
Preferred shares
Class of Stock [Line Items]
Preferred shares, shares issued at beginning of period	0
Preferred shares, shares outstanding at beginning of period	0
Preferred shares issued	6,000
Preferred shares, shares issued at end of period	6,000
Preferred shares, shares outstanding at end of period	6,000
Preferred shares - Shareholders' Equity
Preferred shares, dividend rate, percentage	5.875%
Preferred stock, par or stated value per share	$ 0.175
Preferred stock, liquidation preference, value	$ 25,000
Preferred stock, issue price per share	$ 25,000
Depositary share equivalent
Preferred shares - Shareholders' Equity
Preferred stock, liquidation preference, value	$ 25
Preferred stock, issue price per share	$ 25,000
Prior to redemption date | Preferred shares
Preferred shares - Shareholders' Equity
Preferred stock, redemption price per share	26,000
Prior to redemption date | Depositary share equivalent
Preferred shares - Shareholders' Equity
Preferred stock, redemption price per share	26
On or after June 15, 2021 | Preferred shares
Preferred shares - Shareholders' Equity
Preferred stock, redemption price per share	25,000
On or after June 15, 2021 | Depositary share equivalent
Preferred shares - Shareholders' Equity
Preferred stock, redemption price per share	25
Before redemption date if change in law or capital disqualification event | Preferred shares
Preferred shares - Shareholders' Equity
Preferred stock, redemption price per share	25,000
Before redemption date if change in law or capital disqualification event | Depositary share equivalent
Preferred shares - Shareholders' Equity
Preferred stock, redemption price per share	$ 25