Commitments and Contingencies - Other Commitments (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017 Rig
Other Commitments [Line Items]
Number of drilling rigs under service contract	2
Rig Expires in September 2017 [Member]
Other Commitments [Line Items]
Drilling rig commitments, contract expiration year and month	2018-09
Number of drilling rigs	1
Rig Expires in October 2018 [Member]
Other Commitments [Line Items]
Drilling rig commitments, contract expiration year and month	2018-10
Number of drilling rigs	1