Operating Segments (Details) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Operating segments
Number of operating segments | item													2
Revenue:
Lease rent revenue													$ 119,895	$ 108,046	$ 101,431
Maintenance reserve revenue													57,091	53,396	53,322
Spare parts and equipment sales													17,783	25,582	8,917
Gain on sale of leased equipment													3,482	8,320	5,882
Other revenue													9,023	2,718	4,506
Total revenue	$ 55,476	$ 51,461	$ 49,618	$ 50,719	$ 55,186	$ 57,730	$ 43,810	$ 41,336	$ 41,205	$ 45,519	$ 43,994	$ 43,340	207,274	198,062	174,058
Expenses:
Depreciation expense													66,280	69,424	65,314
Cost Of Spare Parts and Equipment Sales													13,293	17,849	7,474
General and administrative													47,780	42,744	35,859
Interest Expense													41,144	39,012	37,062
Net finance costs													41,281	37,861	37,062
Other expense													16,507	18,584	17,938
Total expenses													185,141	186,462	163,647
Earnings from operations													22,133	11,600	10,411
Total assets	1,337,887				1,294,285				1,245,841				1,337,887	1,294,285	1,245,841
Intersegment Eliminations [Member]
Revenue:
Other revenue													(1,635)	(458)	(736)
Total revenue													(1,635)	(458)	(736)
Expenses:
Earnings from operations													(1,635)	(458)	(736)
Leasing and Related Operations
Revenue:
Lease rent revenue														108,046
Maintenance reserve revenue														53,396
Spare parts and equipment sales														9,975
Gain on sale of leased equipment														8,320
Other revenue														2,517
Total revenue														182,254
Expenses:
Depreciation expense														69,135
Cost Of Spare Parts and Equipment Sales														5,734
General and administrative														39,974
Net finance costs														37,474
Other expense														18,584
Total expenses														170,901
Earnings from operations														11,353
Leasing and Related Operations | Operating Segments [Member]
Revenue:
Lease rent revenue													119,895		101,431
Maintenance reserve revenue													57,091		53,322
Spare parts and equipment sales													3,335
Gain on sale of leased equipment													3,482		5,882
Other revenue													8,711		3,581
Total revenue													192,514		164,216
Expenses:
Depreciation expense													65,939		65,025
Cost Of Spare Parts and Equipment Sales													2,394
General and administrative													44,703		33,211
Interest Expense															36,779
Net finance costs													40,813
Other expense													16,507		17,830
Total expenses													170,356		152,845
Earnings from operations													22,158		11,371
Total assets	1,307,460				1,267,414				1,226,701				1,307,460	1,267,414	1,226,701
Spare Parts Sales
Revenue:
Spare parts and equipment sales														15,607
Other revenue														659
Total revenue														16,266
Expenses:
Depreciation expense														289
Cost Of Spare Parts and Equipment Sales														12,115
General and administrative														2,770
Net finance costs														387
Total expenses														15,561
Earnings from operations														705
Spare Parts Sales | Operating Segments [Member]
Revenue:
Spare parts and equipment sales													14,448		8,917
Other revenue													1,947		1,661
Total revenue													16,395		10,578
Expenses:
Depreciation expense													341		289
Cost Of Spare Parts and Equipment Sales													10,899		7,474
General and administrative													3,077		2,648
Interest Expense															283
Net finance costs													468
Other expense															108
Total expenses													14,785		10,802
Earnings from operations													1,610		(224)
Total assets	$ 30,427				$ 26,871				$ 19,140				$ 30,427	$ 26,871	$ 19,140