Income Tax - Components of Income Tax Expense (Benefit) (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
Currently Payable:
Federal									$ 22,001	$ 18,998	$ 21,221
Deferred:
Federal									9,969	8,233	3,952
Tax Cuts and Jobs Act - Rate Reform Impact									5,120
State									434	378	492
Income Tax Expense	$ 15,210	$ 7,939	$ 7,207	$ 7,168	$ 7,850	$ 7,469	$ 6,716	$ 5,574	$ 37,524	$ 27,609	$ 25,665