Investments (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended												12 Months Ended
	Oct. 31, 2014	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 03, 2014
Investments
Investment in WMES joint ventures at beginning of the period					$ 41,590				$ 23,485					$ 41,590	$ 23,485
Investment														630	17,623	$ 11,219
Earnings (losses) from joint ventures														1,068	1,221
Amortization of deferred gain														107	108
Distribution														(1,304)	(847)
Foreign Currency Translation Adjustment														796
Investment in WMES joint ventures at end of the period		$ 41,295				$ 41,590				$ 23,485				41,295	41,590	23,485
Consolidated Statements of Income
Revenue		55,195	$ 57,758	$ 43,843	42,814	41,563	$ 45,519	$ 43,865	43,340	47,203	$ 37,952	$ 37,953	$ 35,304	199,610	174,287	158,412
Expenses														186,641	163,774	150,638
Net income		2,973	$ 2,577	$ (492)	2,298	(277)	$ 979	$ 2,214	4,331	6,553	$ (2,229)	$ 9,692	$ 1,610	7,356	7,247	15,626
Total assets		1,308,577				1,261,626								1,308,577	1,261,626
Total liabilities		1,098,245				1,044,765								1,098,245	1,044,765
Total liabilities and shareholders' equity		$ 1,308,577				1,261,626								$ 1,308,577	1,261,626
WMES
Investments
Ownership interest (as a percent)		50.00%												50.00%
Investment in WMES joint ventures at beginning of the period					26,672				$ 23,485					$ 26,672	23,485
Investment														630	2,623
Earnings (losses) from joint ventures														1,167	1,303
Amortization of deferred gain														107	108
Distribution														(1,304)	(847)
Investment in WMES joint ventures at end of the period		$ 27,272				26,672				$ 23,485				27,272	26,672	23,485
Management fees														1,700	2,000	1,700
Engine Value		200												200
Consolidated Statements of Income
Revenue														26,909	25,757	16,312
Expenses														24,574	23,150	15,538
Net income														2,335	2,607	774
Total assets		256,126				268,924								256,126	268,924
Total liabilities		195,258				209,044								195,258	209,044
Total liabilities and shareholders' equity		60,868				59,880								60,868	59,880
WMES | Other Revenue
Investments
Management fees														1,700	2,000	$ 1,600
CASC Willis
Investments
Ownership interest (as a percent)																	50.00%
Reduced Investment														14,000
Loss reported														200
Investment in WMES joint ventures at beginning of the period					$ 14,918									14,918
Investment	$ 15,000														15,000
Earnings (losses) from joint ventures														(99)	(82)
Foreign Currency Translation Adjustment														796
Investment in WMES joint ventures at end of the period		$ 14,023				$ 14,918								$ 14,023	$ 14,918