Reportable Segments (Details) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	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, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Reportable Segments
Number of operating segments | item													2
Revenue:
Lease rent revenue													$ 130,369	$ 119,895	$ 108,046
Maintenance reserve revenue													80,189	57,091	53,396
Spare parts and equipment sales													51,423	17,783	25,582
Gain on sale of leased equipment													4,929	3,482	8,320
Other revenue													7,930	9,023	2,718
Total revenue	$ 63,189	$ 65,861	$ 67,844	$ 77,946	$ 55,476	$ 51,461	$ 49,618	$ 50,719	$ 55,186	$ 57,730	$ 43,810	$ 41,336	274,840	207,274	198,062
Expenses:
Depreciation expense													66,023	66,280	69,424
Cost of spare parts and equipment sales													40,848	13,293	17,849
General and administrative													55,737	47,780	42,744
Net finance costs															37,861
Net finance costs													48,720	41,281	37,861
Other expense													34,659	16,507	18,584
Total expenses													245,987	185,141	186,462
Earnings from operations													28,853	22,133	11,600
Total assets	1,603,431				1,337,887				1,294,285				1,603,431	1,337,887	1,294,285
Intersegment Eliminations [Member]
Revenue:
Other revenue													(1,373)	(1,635)	(458)
Total revenue													(1,373)	(1,635)	(458)
Expenses:
Earnings from operations													(1,373)	(1,635)	(458)
Leasing and Related Operations | Operating Segments [Member]
Revenue:
Lease rent revenue													130,369	119,895	108,046
Maintenance reserve revenue													80,189	57,091	53,396
Spare parts and equipment sales													22,285	3,335	9,975
Gain on sale of leased equipment													4,929	3,482	8,320
Other revenue													7,702	8,711	2,517
Total revenue													245,474	192,514	182,254
Expenses:
Depreciation expense													65,677	65,939	69,135
Cost of spare parts and equipment sales													17,344	2,394	5,734
General and administrative													52,024	44,703	39,974
Net finance costs															37,474
Net finance costs													48,720	40,813
Other expense													34,659	16,507	18,584
Total expenses													218,424	170,356	170,901
Earnings from operations													27,050	22,158	11,353
Total assets	1,580,094				1,307,460				1,267,414				1,580,094	1,307,460	1,267,414
Spare Parts Sales | Operating Segments [Member]
Revenue:
Spare parts and equipment sales													29,138	14,448	15,607
Other revenue													1,601	1,947	659
Total revenue													30,739	16,395	16,266
Expenses:
Depreciation expense													346	341	289
Cost of spare parts and equipment sales													23,504	10,899	12,115
General and administrative													3,713	3,077	2,770
Net finance costs															387
Net finance costs														468
Total expenses													27,563	14,785	15,561
Earnings from operations													3,176	1,610	705
Total assets	$ 23,337				$ 30,427				$ 26,871				$ 23,337	$ 30,427	$ 26,871