Commitments and Contingencies (Narrative) (Detail) (USD $)	1 Months Ended	9 Months Ended		9 Months Ended			1 Months Ended
	Sep. 30, 2012 sqft	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Hyundai Heavy Industries Co Ltd [Member] Contract Installment	Sep. 30, 2011 Hyundai Heavy Industries Co Ltd [Member]	Sep. 30, 2012 Keppel AmFELSLLC [Member] Installment	Sep. 30, 2012 Jurong Shipyard Pte Ltd [Member] Installment
Schedule Of Commitments And Contingencies [Line Items]
Deductible for marine liability coverage including personal injury claims, per first occurrence		$ 10,000,000
Range of deductible for liability coverage for personal injury claims, lower limit		5,000,000
Range of deductible for liability coverage for personal injury claims, upper limit		100,000,000
Estimated liability for personal injury claims	36,500,000	36,500,000	32,700,000
Reserve for personal injury claims recorded in accrued liabilities	10,400,000	10,400,000	10,100,000
Reserve for personal injury claims recorded in other liabilities	26,100,000	26,100,000	22,600,000
Total water depth in feet for new moored semisubmersible rig	6,000
Total unconditional purchase obligation						300,000,000	370,000,000
Total number of installments due under vessel modification agreement						11	12
Vessel modification agreement Payment during the period						65,700,000	13,500,000
Number of turnkey contracts for drillship construction				4
Total unconditional purchase obligation for drillships				2,600,000,000
Total number of installments due under each drillship turnkey construction agreement				2
First Installments on Drillships aggregate amount				169,300,000	478,300,000
Other Purchase obligations	0	0	0
Contingent liability under letters of credit and other bonds	126,100,000	126,100,000
Performance bonds that may require collateral at any time	$ 111,100,000	$ 111,100,000