Shareholders' equity	12 Months Ended
Dec. 31, 2017
Equity [Abstract]
Shareholders' equity	Shareholders’ equity (a) Dual common share capitalization The Company has been capitalized with class A common shares, of which there are 240,000,000 authorized, and class B common shares, of which there are 120,000,000 authorized, each convertible at any time into one class A common share. In general, holders of class A and class B common shares vote together as a single class, with holders of class B shares having one vote per share and holders of class A shares having one tenth of one vote per share. In all other substantial respects, the class A and class B common shares are the same. (b) Shareholder rights agreement The Company has in place a shareholder rights agreement which will be implemented not earlier than the tenth day following the first to occur of (i) the public announcement of the acquisition by a person (other than a subsidiary of the Company) of 15% or more of the outstanding class A common shares or 15% or more of the outstanding class B common shares, and (ii) the commencement or announcement of a tender offer or exchange offer by a person for 30% or more of the outstanding class A common shares or 30% or more of the outstanding class B common shares. At that time, the rights will detach from the class A and class B common shares, and the holders of the rights will be entitled to purchase, for each right held, one one hundredth of a series A junior participating preferred share of the Company at an exercise price of $70 (the “Purchase Price”) for each one one hundredth of such junior preferred share, subject to adjustment in certain events. From and after the date on which any person acquires beneficial ownership of 15% or more of the outstanding class A common shares or 15% or more of the outstanding class B common shares, each holder of a right (other than the acquiring person) will be entitled upon exercise to receive, at the then current Purchase Price and in lieu of the junior preferred shares, that number of class A or class B common shares (depending on whether the right was previously attached to a class A or B share) having a market value of twice the Purchase Price. If the Company is acquired or 50% or more of its consolidated assets or earning power is sold, each holder of a right will be entitled to receive, upon exercise at the then current Purchase Price, that amount of common equity of the acquiring company which at the time of such transaction would have a market value of two times the Purchase Price. Also, the Company’s board of directors may exchange all or some of the rights for class A and class B common shares (depending on whether the right was previously attached to a class A or B share) if any person acquires 15% beneficial ownership as described above, but less than 50% beneficial ownership. The rights will expire on June 1, 2020 but may be redeemed at a price of $0.05 per right at any time prior to the tenth day following the date on which a person acquires beneficial ownership of 15% or more of the outstanding class A common shares or 15% or more of the outstanding class B common shares. (c) Acquired shares Included in shareholders’ equity is a reduction for 18,044,478 class B common shares of the Company that a subsidiary of the Company acquired in 2002. Under applicable Bermuda law, these shares are outstanding and may be voted, although in computing earnings per share these shares are treated as a reduction to outstanding shares. (d) Preferred shares The Company has 30,000,000 authorized preferred shares, par value $0.01 each, 500,000 of which have been reserved for issuance as series A junior participating preferred shares upon exercise of preferred share purchase rights held by class A and B common shareholders in connection with the shareholder rights agreement.

Shareholders’ equity

(a) Dual common share capitalization

The Company has been capitalized with class A common shares, of which there are 240,000,000 authorized, and class B common shares, of which there are 120,000,000 authorized, each convertible at any time into one class A common share. In general, holders of class A and class B common shares vote together as a single class, with holders of class B shares having one vote per share and holders of class A shares having one tenth of one vote per share. In all other substantial respects, the class A and class B common shares are the same.

(b) Shareholder rights agreement

The Company has in place a shareholder rights agreement which will be implemented not earlier than the tenth day following the first to occur of (i) the public announcement of the acquisition by a person (other than a subsidiary of the Company) of 15% or more of the outstanding class A common shares or 15% or more of the outstanding class B common shares, and (ii) the commencement or announcement of a tender offer or exchange offer by a person for 30% or more of the outstanding class A common shares or 30% or more of the outstanding class B common shares. At that time, the rights will detach from the class A and class B common shares, and the holders of the rights will be entitled to purchase, for each right held, one one hundredth of a series A junior participating preferred share of the Company at an exercise price of $70 (the “Purchase Price”) for each one one hundredth of such junior preferred share, subject to adjustment in certain events. From and after the date on which any person acquires beneficial ownership of 15% or more of the outstanding class A common shares or 15% or more of the outstanding class B common shares, each holder of a right (other than the acquiring person) will be entitled upon exercise to receive, at the then current Purchase Price and in lieu of the junior preferred shares, that number of class A or class B common shares (depending on whether the right was previously attached to a class A or B share) having a market value of twice the Purchase Price. If the Company is acquired or 50% or more of its consolidated assets or earning power is sold, each holder of a right will be entitled to receive, upon exercise at the then current Purchase Price, that amount of common equity of the acquiring company which at the time of such transaction would have a market value of two times the Purchase Price. Also, the Company’s board of directors may exchange all or some of the rights for class A and class B common shares (depending on whether the right was previously attached to a class A or B share) if any person acquires 15% beneficial ownership as described above, but less than 50% beneficial ownership. The rights will expire on June 1, 2020 but may be redeemed at a price of $0.05 per right at any time prior to the tenth day following the date on which a person acquires beneficial ownership of 15% or more of the outstanding class A common shares or 15% or more of the outstanding class B common shares.

Included in shareholders’ equity is a reduction for 18,044,478 class B common shares of the Company that a subsidiary of the Company acquired in 2002. Under applicable Bermuda law, these shares are outstanding and may be voted, although in computing earnings per share these shares are treated as a reduction to outstanding shares.

(d) Preferred shares

The Company has 30,000,000 authorized preferred shares, par value $0.01 each, 500,000 of which have been reserved for issuance as series A junior participating preferred shares upon exercise of preferred share purchase rights held by class A and B common shareholders in connection with the shareholder rights agreement.