Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale
Amortized Cost	$ 2,484,948	$ 2,580,520
Gross Unrealized Gains	45,399	23,360
Gross Unrealized Losses	(8,025)	(32,217)
Fair Value	2,522,322	2,571,663
Held-to-maturity
Amortized Cost	255,572	297,915
Gross Unrealized Gains	5,842	1,380
Gross Unrealized Losses	(6)	(11,039)
Fair Value	261,408	288,256
Total
Amortized Cost	2,740,520	2,878,435
Gross Unrealized Gains	51,241	24,740
Gross Unrealized Losses	(8,031)	(43,256)
Fair Value	2,783,730	2,859,919
U.S. government and federal agency
Available-for-sale
Amortized Cost	21,685	23,757
Gross Unrealized Gains	22	54
Gross Unrealized Losses	(186)	(162)
Fair Value	21,521	23,649
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	105,189	120,670
Gross Unrealized Gains	1,269	52
Gross Unrealized Losses	(1)	(514)
Fair Value	106,457	120,208
State and local governments
Available-for-sale
Amortized Cost	700,070	844,636
Gross Unrealized Gains	25,426	18,936
Gross Unrealized Losses	(305)	(11,322)
Fair Value	725,191	852,250
Held-to-maturity
Amortized Cost	255,572	297,915
Gross Unrealized Gains	5,842	1,380
Gross Unrealized Losses	(6)	(11,039)
Fair Value	261,408	288,256
Corporate bonds
Available-for-sale
Amortized Cost	217,494	292,052
Gross Unrealized Gains	1,293	378
Gross Unrealized Losses	(239)	(1,613)
Fair Value	218,548	290,817
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	828,103	808,537
Gross Unrealized Gains	3,154	628
Gross Unrealized Losses	(6,209)	(16,250)
Fair Value	825,048	792,915
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	612,407	490,868
Gross Unrealized Gains	14,235	3,312
Gross Unrealized Losses	(1,085)	(2,356)
Fair Value	$ 625,557	$ 491,824