Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale
Amortized Cost	$ 2,394,617	$ 2,580,520
Gross Unrealized Gains	65,598	23,360
Gross Unrealized Losses	(1,179)	(32,217)
Fair Value	2,459,036	2,571,663
Held-to-maturity
Amortized Cost	234,992	297,915
Gross Unrealized Gains	10,567	1,380
Gross Unrealized Losses	0	(11,039)
Fair Value	245,559	288,256
Total
Amortized Cost	2,629,609	2,878,435
Gross Unrealized Gains	76,165	24,740
Gross Unrealized Losses	(1,179)	(43,256)
Fair Value	2,704,595	2,859,919
U.S. government and federal agency
Available-for-sale
Amortized Cost	147,575	23,757
Gross Unrealized Gains	26	54
Gross Unrealized Losses	(167)	(162)
Fair Value	147,434	23,649
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	66,072	120,670
Gross Unrealized Gains	1,117	52
Gross Unrealized Losses	0	(514)
Fair Value	67,189	120,208
State and local governments
Available-for-sale
Amortized Cost	588,307	844,636
Gross Unrealized Gains	25,558	18,936
Gross Unrealized Losses	0	(11,322)
Fair Value	613,865	852,250
Held-to-maturity
Amortized Cost	234,992	297,915
Gross Unrealized Gains	10,567	1,380
Gross Unrealized Losses	0	(11,039)
Fair Value	245,559	288,256
Corporate bonds
Available-for-sale
Amortized Cost	145,528	292,052
Gross Unrealized Gains	1,860	378
Gross Unrealized Losses	(5)	(1,613)
Fair Value	147,383	290,817
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	762,415	808,537
Gross Unrealized Gains	5,662	628
Gross Unrealized Losses	(824)	(16,250)
Fair Value	767,253	792,915
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	684,720	490,868
Gross Unrealized Gains	31,375	3,312
Gross Unrealized Losses	(183)	(2,356)
Fair Value	$ 715,912	$ 491,824