Income Tax Expense (Schedule of Components of Income Tax Expense) (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
Current tax expense:
Federal									$ 23,827	$ 19,444	$ 17,864
State									2,201	2,061	1,194
Total current tax expense									26,028	21,505	19,058
Deferred tax expense (benefit):
Federal									5,717	796	2,003
State									(30)	72	(183)
Total deferred tax expense									5,687	868	1,820
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