Commitments and Contingent Liabilities (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) drilling_unit	Dec. 31, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Letters of credit issued	$ 7.7
IRS proposed unfavorable tax adjustments	85.0
Gross unrecognized tax benefits	98.0	$ 102.0
Tax impact of net unrecognized tax benefits, if reversed	$ 35.0
Saudi Arabia Joint Venture [Member]
Long-term Purchase Commitment [Line Items]
Period Of Joint Venture Funding Of Drilling Rigs	10 years
Saudi Arabia Joint Venture [Member] | Maximum [Member]
Long-term Purchase Commitment [Line Items]
Number Of Drilling Rigs To Be Purchased By Joint Venture | drilling_unit	20
Contingent Funding Obligation To Equity Method Investment	$ 1,250.0
Number Of Drilling Rigs To Be Purchased By Each Partner in a 12 month period | drilling_unit	3
New Revolving Credit Facility [Member]
Long-term Purchase Commitment [Line Items]
Letters of credit issued	$ 5.4