Income Taxes (Valuation Allowance) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income tax expense	$ 5,339	$ 5,127	$ 4,414	$ 3,852	$ 3,336	$ 3,656	$ 3,479	$ 3,654	$ 18,732	$ 14,125	$ 3,875
Deferred Tax Assets, Gross	11,792				$ 20,827				11,792	$ 20,827
Partnership investment
Deferred Tax Assets, Gross	$ 3,800								3,800
Adjustments for New Accounting Pronouncement [Member]
Income tax expense									$ 900