Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale
Amortized Cost	$ 2,403,517	$ 2,580,520
Gross Unrealized Gains	69,191	23,360
Gross Unrealized Losses	(2,074)	(32,217)
Fair Value	2,470,634	2,571,663
Held-to-maturity
Amortized Cost	252,097	297,915
Gross Unrealized Gains	8,597	1,380
Gross Unrealized Losses	0	(11,039)
Fair Value	260,694	288,256
Total
Amortized Cost	2,655,614	2,878,435
Gross Unrealized Gains	77,788	24,740
Gross Unrealized Losses	(2,074)	(43,256)
Fair Value	2,731,328	2,859,919
U.S. government and federal agency
Available-for-sale
Amortized Cost	19,603	23,757
Gross Unrealized Gains	25	54
Gross Unrealized Losses	(141)	(162)
Fair Value	19,487	23,649
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	117,911	120,670
Gross Unrealized Gains	2,039	52
Gross Unrealized Losses	0	(514)
Fair Value	119,950	120,208
State and local governments
Available-for-sale
Amortized Cost	671,714	844,636
Gross Unrealized Gains	28,506	18,936
Gross Unrealized Losses	(27)	(11,322)
Fair Value	700,193	852,250
Held-to-maturity
Amortized Cost	252,097	297,915
Gross Unrealized Gains	8,597	1,380
Gross Unrealized Losses	0	(11,039)
Fair Value	260,694	288,256
Corporate bonds
Available-for-sale
Amortized Cost	177,812	292,052
Gross Unrealized Gains	1,895	378
Gross Unrealized Losses	(23)	(1,613)
Fair Value	179,684	290,817
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	697,041	808,537
Gross Unrealized Gains	5,432	628
Gross Unrealized Losses	(1,684)	(16,250)
Fair Value	700,789	792,915
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	719,436	490,868
Gross Unrealized Gains	31,294	3,312
Gross Unrealized Losses	(199)	(2,356)
Fair Value	$ 750,531	$ 491,824