Selected Financial Data (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 76,589	$ 73,627	$ 71,581	$ 68,029	$ 67,855	$ 66,723	$ 64,914	$ 61,150	$ 289,826	$ 260,642	$ 227,774
Interest expense	16,747	14,365	13,178	11,097	10,027	9,267	8,344	7,272	55,388	34,909	24,515
Provision for loan losses	2,716	462	9,345	2,472	982	2,850	4,869	5,183	14,995	13,883	17,965
Net Interest Income After Provision for Loan Losses	57,126	58,800	49,058	54,460	56,846	54,606	51,701	48,695	219,443	211,850	185,294
Available-for-sale Securities, Gross Realized Gain (Loss)	0	0	0	0	(986)	0	3,617	370
Noninterest income	11,095	12,042	12,251	13,792	13,636	13,551	12,648	12,626	49,181	55,462	54,635
Noninterest expense	36,415	37,085	35,863	36,082	37,947	36,553	36,597	36,808	145,445	147,907	143,232
Income Before Taxes	31,806	33,757	25,446	32,170	31,549	31,604	31,369	24,883
Provision for income taxes	4,952	2,876	4,010	6,007	22,255	8,883	8,604	6,695	17,845	46,437	25,305
Net Income Allocated to Common Shareholders	$ 26,854	$ 30,881	$ 21,436	$ 26,163	$ 9,294	$ 22,721	$ 22,765	$ 18,188	$ 105,030	$ 72,726	$ 71,167
Per Share Data
Common earnings per share-diluted (in USD per share)	$ 0.77	$ 0.88	$ 0.61	$ 0.75	$ 0.27	$ 0.65	$ 0.65	$ 0.52	$ 3.01	$ 2.09	$ 2.05
Dividends declared per common share (in USD per share)	0.27	0.25	0.25	0.22	0.22	0.20	0.20	0.20	$ 0.99	$ 0.82	$ 0.77
Common book value (in USD per share)	$ 26.98	$ 26.27	$ 25.91	$ 25.58	$ 25.28	$ 25.37	$ 24.90	$ 24.45