Income Taxes Federal Statutory Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income before income taxes	$ 96,493	$ 86,381	$ 61,154
Income Tax Statutory Rate Reconciliation [Abstract]
“Expected” federal income tax	33,773	30,233	21,404
Increase (decrease) in taxes attributable to:
State taxes, net of federal income tax benefit	4,986	4,874	3,332
Dividend received deduction	(18)	(21)	(43)
Uncertain tax positions	12	16	3
Sales & Use Enterprise Zone Credit	(9)	(117)	0
Tangible Property Regulations	(1,159)	(1,184)	(1,081)
Tax reform - rate change impact on deferred taxes	(2,357)	0	0
Noncontrolling interest income	(664)	0	0
Other items, net	829	(259)	(343)
Income Tax Expense (Benefit)	$ 35,393	$ 33,542	$ 23,272