Securities, Available for Sale (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Position	Dec. 31, 2016 USD ($) Position	Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 1,266,165	$ 1,341,059
Unrealized gains	5,893	11,097
Unrealized losses	16,133	13,866
Estimated fair value	1,255,925	1,338,290
Amortized costs of securities available for sale pledged to secure public deposits	1,500,000	1,500,000
Amortized costs of securities available for sale pledged as collateral for repurchase agreements	231,300	235,600
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	633,766	902,506
12 months or longer	386,818	5,831
Total	1,020,584	908,337
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(5,481)	(12,347)
12 months or longer	(10,653)	(1,519)
Total	$ (16,134)	$ (13,866)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	134	171
12 months or longer | Position	79	7
Total | Position	213	178
Sales transactions of securities available for sale [Abstract]
Gross realized gains	$ 2,241	$ 683	$ 3,099
Gross realized (losses)	(372)	(1,327)	(12)
Net AFS realized gains (losses)	1,869	(644)	3,087
Gains from calls on securities available for sale	100	100	$ 100
Held-to-maturity securities sold, unrealized loss	(2)	$ 0
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Within one year	63,309
From one to five years	90,119
From five to ten years	178,128
After ten years	923,987
Amortized cost	1,255,543
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Within one year	63,186
From one to five years	89,275
From five to ten years	177,961
After ten years	911,354
Fair value	$ 1,241,776
Consolidated stockholders' equity threshold percentage that no single issuer of securities exceeded except for U.S. Government securities	10.00%	10.00%
Realized OTTI impairment loss on AFS equity investment	$ 1,300
Held-to-maturity securities sold, amortized cost	800
Federal Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	109,862	$ 175,135
Unrealized gains	0	78
Unrealized losses	963	805
Estimated fair value	108,899	174,408
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	64,653	119,363
12 months or longer	44,246	0
Total	108,899	119,363
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(242)	(805)
12 months or longer	(721)	0
Total	$ (963)	$ (805)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	5	10
12 months or longer | Position	4	0
Total | Position	9	10
State & Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 42,171	$ 47,053
Unrealized gains	62	153
Unrealized losses	277	480
Estimated fair value	41,956	46,726
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	23,566	31,873
12 months or longer	5,994	483
Total	29,560	32,356
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(200)	(478)
12 months or longer	(77)	(2)
Total	$ (277)	$ (480)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	39	55
12 months or longer | Position	8	1
Total | Position	47	56
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 317,630	$ 277,524
12 months or longer	58,316	985
Total	375,946	278,509
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(2,381)	(2,668)
12 months or longer	(1,188)	(13)
Total	$ (3,569)	$ (2,681)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	55	49
12 months or longer | Position	24	4
Total | Position	79	53
Mortgage-Backed, Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 530,392	$ 513,814
Unrealized gains	1,406	3,345
Unrealized losses	3,345	2,492
Estimated fair value	528,453	514,667
Mortgage-backed Securities, U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	26,363	14,955
Unrealized gains	334	411
Unrealized losses	223	189
Estimated fair value	26,474	15,177
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	227,917	473,746
12 months or longer	275,303	0
Total	503,220	473,746
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(2,658)	(8,396)
12 months or longer	(8,521)	0
Total	$ (11,179)	$ (8,396)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	35	57
12 months or longer | Position	42	0
Total | Position	77	57
Collateralized Mortgage Obligations, Government-Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 496,033	$ 513,431
Unrealized gains	254	532
Unrealized losses	10,114	7,688
Estimated fair value	486,173	506,275
Collateralized Mortgage Obligations, U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	50,721	60,822
Unrealized gains	165	184
Unrealized losses	1,065	708
Estimated fair value	49,821	60,298
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	10,623	15,849
Unrealized gains	3,672	6,394
Unrealized losses	146	1,504
Estimated fair value	14,149	20,739
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	0	0
12 months or longer	2,959	4,363
Total	2,959	4,363
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	0	0
12 months or longer	(146)	(1,504)
Total	$ (146)	$ (1,504)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	0	0
12 months or longer | Position	1	2
Total | Position	1	2