Income Tax Expense (Schedule of Income Tax Rate Reconciliation) (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
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Tax expense at Federal statutory rate									$ 27,174	$ 24,099	$ 22,520
(Decrease) increase in taxes resulting from:
Tax-exempt income									(1,313)	(1,599)	(1,604)
Dividends received deduction									(55)	(58)	(57)
BOLI									(757)	(930)	(694)
Stock based compensation									(481)	0
Federal tax credits									(364)	(364)	(364)
Change in fair value of contingent consideration									(225)	(314)	0
Acquisition related expenses									0	0	318
State income tax expense, net of federal income tax benefit									1,411	1,387	658
Adjustment to net deferred tax asset for enacted changes in federal tax law									6,170	0	0
Other									155	152	101
Total income tax expense	$ 13,163	$ 6,326	$ 6,505	$ 5,721	$ 5,873	$ 5,863	$ 5,153	$ 5,484	$ 31,715	$ 22,373	$ 20,878