Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 165,773	$ 172,417	$ 173,466	$ 173,130	$ 172,026	$ 167,826	$ 159,910	$ 150,410	$ 684,786	$ 650,172	$ 572,104
Interest expense	52,265	56,159	56,245	53,522	50,133	44,679	37,091	30,342	218,191	162,245	90,724
NET INTEREST INCOME BEFORE PROVISION FOR LOAN LOSSES	113,508	116,258	117,221	119,608	121,893	123,147	122,819	120,068	466,595	487,927	481,380
Provision for loan losses	1,000	2,100	1,200	3,000	2,800	7,300	2,300	2,500	7,300	14,900	17,360
NET INTEREST INCOME AFTER PROVISION FOR LOAN LOSSES	112,508	114,158	116,021	116,608	119,093	115,847	120,519	117,568	459,295	473,027	464,020
Noninterest income	12,979	12,995	12,287	11,422	11,614	13,447	15,269	19,850	49,683	60,180	66,415
Noninterest expense	70,429	69,995	71,371	70,833	70,189	67,455	71,629	68,453	282,628	277,726	266,601
INCOME BEFORE INCOME TAX PROVISION	55,058	57,158	56,937	57,197	60,518	61,839	64,159	68,965	226,350	255,481	263,834
Income tax provision	12,049	14,566	14,256	14,439	16,069	15,461	16,629	17,733	55,310	65,892	124,389
NET INCOME	$ 43,009	$ 42,592	$ 42,681	$ 42,758	$ 44,449	$ 46,378	$ 47,530	$ 51,232	$ 171,040	$ 189,589	$ 139,445
Basic EPS - common stock (in dollars per share)	$ 0.34	$ 0.34	$ 0.34	$ 0.34	$ 0.35	$ 0.36	$ 0.36	$ 0.38	$ 1.35	$ 1.44	$ 1.03
Diluted EPS - common stock (in dollars per share)	$ 0.34	$ 0.34	$ 0.34	$ 0.34	$ 0.35	$ 0.36	$ 0.36	$ 0.38	$ 1.35	$ 1.44	$ 1.03