Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale
Amortized Cost	$ 2,580,520	$ 1,771,504
Gross Unrealized Gains	23,360	22,433
Gross Unrealized Losses	(32,217)	(15,694)
Total	2,571,663	1,778,243
Held-to-maturity
Amortized Cost	297,915	648,313
Gross Unrealized Gains	1,380	20,346
Gross Unrealized Losses	(11,039)	(8,573)
Fair Value	288,256	660,086
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Amortized Cost	2,878,435	2,419,817
Gross Unrealized Gains	24,740	42,779
Gross Unrealized Losses	(43,256)	(24,267)
Fair Value	2,859,919	2,438,329
U.S. government and federal agency
Available-for-sale
Amortized Cost	23,757	31,216
Gross Unrealized Gains	54	54
Gross Unrealized Losses	(162)	(143)
Total	23,649	31,127
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	120,670	19,195
Gross Unrealized Gains	52	0
Gross Unrealized Losses	(514)	(104)
Total	120,208	19,091
State and local governments
Available-for-sale
Amortized Cost	844,636	614,366
Gross Unrealized Gains	18,936	20,299
Gross Unrealized Losses	(11,322)	(5,164)
Total	852,250	629,501
Held-to-maturity
Amortized Cost	297,915	648,313
Gross Unrealized Gains	1,380	20,346
Gross Unrealized Losses	(11,039)	(8,573)
Fair Value	288,256	660,086
Corporate bonds
Available-for-sale
Amortized Cost	292,052	216,443
Gross Unrealized Gains	378	802
Gross Unrealized Losses	(1,613)	(483)
Total	290,817	216,762
Residential mortgage backed securities
Available-for-sale
Amortized Cost	808,537	785,960
Gross Unrealized Gains	628	1,253
Gross Unrealized Losses	(16,250)	(7,930)
Total	792,915	779,283
Commercial mortgage backed securities
Available-for-sale
Amortized Cost	490,868	104,324
Gross Unrealized Gains	3,312	25
Gross Unrealized Losses	(2,356)	(1,870)
Total	$ 491,824	$ 102,479