Share capital (Preferred shares) (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Jun. 12, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Class of Stock [Line Items]
Preferred shares issued at beginning of year | shares		6,000
Preferred shares issued | shares		0	0	639
Preferred shares issued at end of year | shares		16,000	6,000
Net proceeds on issuance of preferred shares | $		$ 241,686	$ 144,852	$ 0
Preferred shares
Class of Stock [Line Items]
Preferred shares issued at beginning of year | shares		6,000	0
Preferred shares issued | shares		10,000	6,000
Preferred shares issued at end of year | shares		16,000	6,000	0
Series A Preferred Shares
Class of Stock [Line Items]
Preferred shares issued at beginning of year | shares		6,000
Preferred shares issued at end of year | shares		6,000	6,000
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
Net proceeds on issuance of preferred shares | $		$ 144,852
Series A Depositary Share Equivalent
Class of Stock [Line Items]
Proportionate Interest of Series A Preference Shares, Per Depositary Share		0.001
Preferred shares issued at end of year | shares		6,000,000
Preferred Stock, Liquidation Preference Per Share		$ 25
Series B Preferred Shares
Class of Stock [Line Items]
Preferred shares issued | shares	10,000
Preferred shares issued at end of year | shares		10,000
Preferred Stock, Dividend Rate, Percentage	5.80%
Preferred Stock, Par or Stated Value Per Share	$ 0.175
Preferred Stock, Liquidation Preference Per Share	$ 25,000
Net proceeds on issuance of preferred shares | $	$ 241,686
Series B Depositary Share Equivalent
Class of Stock [Line Items]
Proportionate Interest of Series A Preference Shares, Per Depositary Share		0.001
Preferred Shares Equivalent Depositary Shares Number | shares	10,000,000
Preferred Stock, Liquidation Preference Per Share	$ 25
On or after June 15, 2021 | Series A Preferred Shares
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share		$ 25,000
On or after June 15, 2021 | Series A Depositary Share Equivalent
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share		25
On or after June 21, 2022 | Series B Preferred Shares
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	25,000
On or after June 21, 2022 | Series B 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 Shares
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share		25,000
Prior to redemption if change in law or capital disqualification event | Series A 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 B Preferred Shares
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	26,000
Prior to redemption if change in law or capital disqualification event | Series B Depositary Share Equivalent
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	26
Prior to redemption date
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share		26
Prior to redemption date | Series A Preferred Shares
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share		$ 26,000
Prior to redemption date | Series B Preferred Shares
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	25,000
Prior to redemption date | Series B Depositary Share Equivalent
Class of Stock [Line Items]
Preferred Stock, Redemption Price Per Share	$ 25