Securities, Held to Maturity (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($) Position	Jun. 30, 2017 USD ($) Position	Dec. 31, 2016 USD ($) Position	Jun. 30, 2016 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 515,628	$ 515,628	$ 527,948
Unrealized gains	3,507	3,507	1,581
Unrealized losses	2,479	2,479	4,479
Estimated fair value	516,656	516,656	525,050
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	117,750	117,750	286,063
12 months or longer	35,657	35,657	35,209
Total	153,407	153,407	321,272
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(1,423)	(1,423)	(3,290)
12 months or longer	(1,056)	(1,056)	(1,189)
Total	$ (2,479)	$ (2,479)	$ (4,479)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	46	46	172
12 months or longer | Position	9	9	4
Total | Position	55	55	176
Held-to-maturity securities sold, amortized cost	$ 800	$ 800		$ 0
Held-to-maturity securities sold, unrealized loss	(2)	(2)
Amortized costs of securities held to maturity pledged to secure public deposits	1,500,000	1,500,000	$ 1,500,000
Amortized costs of securities held to maturity pledged as collateral for repurchase agreements	234,400	234,400	235,600
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Within one year	34,486	34,486
From one to five years	38,026	38,026
From five to ten years	122,400	122,400
After ten years	320,716	320,716
Amortized cost	515,628	515,628	527,948
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Within one year	34,493	34,493
From one to five years	38,392	38,392
From five to ten years	123,545	123,545
After ten years	320,226	320,226
Fair value	516,656	516,656	525,050
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	52,892	52,892	95,492
12 months or longer	0	0	0
Total	52,892	52,892	95,492
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(878)	(878)	(1,176)
12 months or longer	0	0	0
Total	$ (878)	$ (878)	$ (1,176)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	4	4	5
12 months or longer | Position	0	0	0
Total | Position	4	4	5
Mortgage-Backed, Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 102,651	$ 102,651	$ 96,668
Unrealized gains	61	61	0
Unrealized losses	878	878	1,176
Estimated fair value	101,834	101,834	95,492
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	102,651	102,651	96,668
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	101,834	101,834	95,492
Mortgage-Backed, U.S. Government Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	474	474	533
Unrealized gains	69	69	87
Unrealized losses	0	0	0
Estimated fair value	543	543	620
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	474	474	533
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	543	543	620
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	40,458	40,458	108,587
12 months or longer	32,969	32,969	35,209
Total	73,427	73,427	143,796
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(148)	(148)	(319)
12 months or longer	(920)	(920)	(1,189)
Total	$ (1,068)	$ (1,068)	$ (1,508)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	6	6	12
12 months or longer | Position	4	4	4
Total | Position	10	10	16
Collateralized Mortgage Obligations, Government-Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 205,806	$ 205,806	$ 225,213
Unrealized gains	1,280	1,280	1,060
Unrealized losses	1,068	1,068	1,508
Estimated fair value	206,018	206,018	224,765
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	205,806	205,806	225,213
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	206,018	206,018	224,765
State & Municipal [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	206,697	206,697	205,534
Unrealized gains	2,097	2,097	434
Unrealized losses	533	533	1,795
Estimated fair value	208,261	208,261	204,173
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	24,400	24,400	81,984
12 months or longer	2,688	2,688	0
Total	27,088	27,088	81,984
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(397)	(397)	(1,795)
12 months or longer	(136)	(136)	0
Total	$ (533)	$ (533)	$ (1,795)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	36	36	155
12 months or longer | Position	5	5	0
Total | Position	41	41	155
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	$ 206,697	$ 206,697	$ 205,534
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	$ 208,261	$ 208,261	$ 204,173