Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale
Amortized Cost	$ 2,521,374	$ 2,580,520
Gross Unrealized Gains	55,965	23,360
Gross Unrealized Losses	(2,087)	(32,217)
Total	2,575,252	2,571,663
Held-to-maturity
Amortized Cost	224,611	297,915
Gross Unrealized Gains	9,785	1,380
Gross Unrealized Losses	0	(11,039)
Fair Value	234,396	288,256
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Amortized Cost	2,745,985	2,878,435
Gross Unrealized Gains	65,750	24,740
Gross Unrealized Losses	(2,087)	(43,256)
Fair Value	2,809,648	2,859,919
U.S. government and federal agency
Available-for-sale
Amortized Cost	20,061	23,757
Gross Unrealized Gains	48	54
Gross Unrealized Losses	(65)	(162)
Total	20,044	23,649
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	42,724	120,670
Gross Unrealized Gains	953	52
Gross Unrealized Losses	0	(514)
Total	43,677	120,208
State and local governments
Available-for-sale
Amortized Cost	679,784	844,636
Gross Unrealized Gains	22,694	18,936
Gross Unrealized Losses	(80)	(11,322)
Total	702,398	852,250
Held-to-maturity
Amortized Cost	224,611	297,915
Gross Unrealized Gains	9,785	1,380
Gross Unrealized Losses	0	(11,039)
Fair Value	234,396	288,256
Corporate bonds
Available-for-sale
Amortized Cost	155,665	292,052
Gross Unrealized Gains	1,938	378
Gross Unrealized Losses	(1)	(1,613)
Total	157,602	290,817
Residential mortgage backed securities
Available-for-sale
Amortized Cost	731,766	808,537
Gross Unrealized Gains	7,507	628
Gross Unrealized Losses	(549)	(16,250)
Total	738,724	792,915
Commercial mortgage backed securities
Available-for-sale
Amortized Cost	891,374	490,868
Gross Unrealized Gains	22,825	3,312
Gross Unrealized Losses	(1,392)	(2,356)
Total	$ 912,807	$ 491,824