Income Taxes (Effective Income Tax Rate Reconciliation) (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
Income Tax Disclosure [Abstract]
Federal corporate tax rate										35.00%	35.00%
Computed expected tax on net income									$ 30,495	$ 41,682	$ 40,917
Increase (decrease) resulting from:
Nontaxable interest income									(4,423)	(9,282)	(7,960)
State income taxes, net of federal tax benefit									6,976	3,846	4,768
Tax credits									(4,085)	(2,390)	(1,375)
Valuation allowance									23	405	368
Excess tax benefit on stock compensation									(657)	(1,130)	0
Deferred tax adjustment due to Tax Cuts and Jobs Act enactment									0	10,396	0
Other									(114)	293	(162)
Total income tax expense	$ 6,685	$ 8,956	$ 7,451	$ 5,123	$ 21,506	$ 8,725	$ 8,059	$ 5,530	$ 28,215	$ 43,820	$ 36,556
Effective tax rates									19.40%	36.80%	31.30%