Income Taxes (Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Computed "expected" tax on net income									$ 38,665	$ 30,495	$ 41,682
Increase (decrease) resulting from:
Nontaxable interest income									(3,281)	(4,423)	(9,282)
State income taxes, net of federal tax benefit									8,509	6,976	3,846
Tax credits									(6,860)	(4,085)	(2,390)
Valuation allowance									(1,648)	23	405
Excess tax benefit on stock compensation									(229)	(657)	(1,130)
Deferred tax adjustment due to Tax Cuts and Jobs Act enactment									0	0	10,396
Other									(166)	(114)	293
Total income tax expense	$ 5,155	$ 7,941	$ 13,584	$ 8,310	$ 6,685	$ 8,956	$ 7,451	$ 5,123	$ 34,990	$ 28,215	$ 43,820
Effective tax rates									19.00%	19.40%	36.80%