SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue and Expense Recognition, Accounts Receivable (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 15, 2016	Dec. 31, 2015
Deferred Revenue [Abstract]
Deferred revenue, current	$ 103.9		$ 33.1
Deferred revenue	114.4		50.8
Deferred Costs [Abstract]
Deferred contract costs	2.2		4.4
Receivables - Trade and Other
Trade	286.2		395.7
Income tax	7.7		4.5
Other	7.4		10.3
Total receivables - trade and other	301.3		410.5
Cobalt International Energy [Member]
Deferred Revenue [Abstract]
Deferred revenue	95.9
Cobalt International Energy [Member] | Rowan Reliance [Member]
Deferred Revenue [Abstract]
Deferred revenue		$ 95.9
Deferred Revenue [Member]
Deferred Revenue [Abstract]
Deferred revenue, current	103.9		33.1
Deferred Revenue [Member] | Cobalt International Energy [Member]
Deferred Revenue [Abstract]
Deferred revenue, current	86.5
Other Liabilities [Member]
Deferred Revenue [Abstract]
Deferred revenue, noncurrent	10.5		17.7
Other Liabilities [Member] | Cobalt International Energy [Member]
Deferred Revenue [Abstract]
Deferred revenue, noncurrent	9.4
Other Liabilities [Member] | Cobalt International Energy [Member] | Rowan Reliance [Member]
Deferred Revenue [Abstract]
Deferred revenue	9.4
Prepaid Expenses and Other Current Assets [Member]
Deferred Costs [Abstract]
Deferred contract costs, current	2.0		3.2
Other Assets [Member]
Deferred Costs [Abstract]
Deferred contract costs, noncurrent	$ 0.2		$ 1.2