Details of Number of Vessels, Total Proceeds, Carrying Values at the Time of Sale, Deferred Gains Recognized, Lease Expirations, and Contractual Purchase Option (Detail) $ in Thousands	3 Months Ended							12 Months Ended
	Mar. 31, 2015 USD ($) Vessel	Dec. 31, 2014 USD ($) Vessel	Sep. 30, 2014 USD ($) Vessel	Jun. 30, 2014 USD ($) Vessel	Mar. 31, 2014 USD ($) Vessel	Dec. 31, 2013 USD ($) Vessel	Sep. 30, 2013 USD ($) Vessel	Mar. 31, 2015 USD ($) Vessel	Mar. 31, 2014 USD ($) Vessel
Sale Leaseback Transaction [Line Items]
Number of Vessels | Vessel	1	3	1	1	4	4	2	6	10
Total Proceeds	$ 13,000	$ 78,200	$ 19,350	$ 13,400	$ 63,305	$ 141,900	$ 65,550	$ 123,950	$ 270,755
Carrying Value at time of Sale	5,115	33,233	8,214	4,002	32,845	105,649	34,325	50,564	172,819
Deferred Gain at time of Sale	$ 7,885	$ 44,967	$ 11,136	$ 9,398	$ 30,460	$ 36,251	$ 31,225	$ 73,386	$ 97,936
Lease Term in Years	7 years		8 years 6 months	7 years			7 years
Purchase Option Percentage	50.00%	60.00%	47.00%	61.00%			55.00%
Purchase Option at at end of:	6 years		8 years	6 years			6 years
Minimum
Sale Leaseback Transaction [Line Items]
Lease Term in Years		8 years			7 years	7 years
Purchase Option Percentage					53.00%	54.00%
Purchase Option at at end of:		7 years			6 years	6 years
Maximum
Sale Leaseback Transaction [Line Items]
Lease Term in Years		9 years			10 years	9 years
Purchase Option Percentage					59.00%	68.00%
Purchase Option at at end of:		8 years			9 years	8 years