Commitments and Contingencies (Details Textuals) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Commitments And Contingencies Line Items [Abstract]
Recognized Gross losses from catastrophic events	$ 175,500,000	$ 44,000,000
Liability related to indemnifications	12,400,000
Pretax loss after reinsurance	117,900,000	21,300,000
Letters of credit to cover obligations or anticipated payments	5,200,000
Amount in escrow account	6,700,000
Recognized Gross Losses For Deepwater Horizon Rig Disaster		31,700,000
Operating Leases, Rent Expense	16,200,000	15,900,000	15,800,000
Terrorism Insurance Deductible	$ 135,500,000
Federal Reimbursement Percentage For Terrorism Losses	85.00%