Debt and financing arrangements (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Schedule of financing structure and finance expenses
Debt instrument, drawn	$ 536,728	$ 536,674
Debt instrument and line of credit facility amount committed, total	1,605,000	1,605,000
Debt instrument and line of credit facility outstanding, total	905,763	901,807
Syndicated unsecured letter of credit facility [Member]
Schedule of financing structure and finance expenses
Debt instrument, drawn	0	0
Letter of credit facility, commitment	340,000	340,000
Credit facility, outstanding	0	0
Bilateral unsecured letter of credit facility [Member]
Schedule of financing structure and finance expenses
Debt instrument, drawn	0	0
Letter of credit facility, commitment	60,000	60,000
Credit facility, outstanding	0	0
Secured letter of credit facility [Member]
Schedule of financing structure and finance expenses
Debt instrument, drawn	0	0
Letter of credit facility, commitment	500,000	500,000
Credit facility, outstanding	277,679	268,944
Talbot Fal Facility [Member]
Schedule of financing structure and finance expenses
Debt instrument, drawn	0	0
Letter of credit facility, commitment	25,000	25,000
Credit facility, outstanding	25,000	25,000
Ipc Bi Lateral Facility [Member]
Schedule of financing structure and finance expenses
Debt instrument, drawn	0	0
Letter of credit facility, commitment	80,000	80,000
Credit facility, outstanding	63,284	68,063
2006 Junior Subordinated Deferrable Debentures [Member]
Schedule of financing structure and finance expenses
Debt instrument, commitment	150,000	150,000
Debt instrument, outstanding	150,000	150,000
Debt instrument, drawn	150,000	150,000
2007 Junior Subordinated Deferrable Debentures [Member]
Schedule of financing structure and finance expenses
Debt instrument, commitment	200,000	200,000
Debt instrument, outstanding	139,800	139,800
Debt instrument, drawn	139,800	139,800
2010 Senior Notes Due 2040 [Member]
Schedule of financing structure and finance expenses
Debt instrument, commitment	250,000	250,000
Debt instrument, outstanding	250,000	250,000
Debt instrument, drawn	$ 246,928	$ 246,874