Share capital (Preferred shares) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Proportionate Interest of Series A Preference Shares, Per Depositary Share	0.10%
Preferred shares issued at beginning of year	0
Preferred shares issued	0	639	1,380
Preferred shares issued at end of year	6,000	0
Net proceeds on issuance of preferred shares	$ 144,852	$ 0	$ 0
Series A Preferred Stock
Class of Stock [Line Items]
Preferred shares issued at beginning of year	0
Preferred shares issued	6,000
Preferred shares issued at end of year	6,000	0
Preferred Stock, Dividend Rate, Percentage	5.875%
Preferred Stock, Par or Stated Value Per Share	$ 0.175
Preferred Stock, Liquidation Preference Per Share	$ 25,000
Depositary Share Equivalent
Class of Stock [Line Items]
Preferred shares issued at end of year	6,000,000
Preferred Stock, Liquidation Preference Per Share	$ 25
On or after June 15, 2021 | Series A Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	25,000
On or after June 15, 2021 | Depositary Share Equivalent
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	25
Prior to redemption if change in law or capital disqualification event | Series A Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	25,000
Prior to redemption if change in law or capital disqualification event | Depositary Share Equivalent
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	25
Prior to redemption date
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	26
Prior to redemption date | Series A Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	$ 26,000