Schedule of Vessel Commitments (Parenthetical) (Detail) $ in Millions	1 Months Ended	3 Months Ended
	Sep. 30, 2015 USD ($) Vessel	Jun. 30, 2016 USD ($) Vessel
Significant Purchase and Supply Commitment [Line Items]
Number of vessels under construction	9
Refunds from cancelled vessel construction contracts | $	$ 31	$ 17
Option Agreement
Significant Purchase and Supply Commitment [Line Items]
Number of vessels under construction		6