Income Tax - Reconciliation of Federal Statutory to Actual Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal Statutory Income Tax at 35%									$ 38,031	$ 31,867	$ 28,800
Tax-exempt Interest Income									(8,749)	(7,083)	(5,148)
Basis Difference on Sale of Insurance Subsidiary										2,252
Stock Compensation									27	34	36
Earnings on Life Insurance									(1,486)	(1,012)	(1,271)
Tax Credits									(564)	(583)	(911)
Other									350	190	617
Income Tax Expense	$ 7,850	$ 7,469	$ 6,716	$ 5,574	$ 4,418	$ 6,557	$ 8,856	$ 5,834	$ 27,609	$ 25,665	$ 22,123
Federal statutory income tax rate									35.00%	35.00%	35.00%