Income Tax - Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Currently Payable:
Federal									$ 23,633	$ 22,001	$ 18,998
State									1,842
Deferred:
Federal									6,723	9,969	8,233
Tax Cuts and Jobs Act - Rate Reform Impact										5,120
State									(3,199)	434	378
Income Tax Expense	$ 5,949	$ 8,478	$ 7,961	$ 6,611	$ 15,210	$ 7,939	$ 7,207	$ 7,168	$ 28,999	$ 37,524	$ 27,609