Securities, Held to Maturity (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($) Position	Sep. 30, 2017 USD ($) Position	Dec. 31, 2016 USD ($) Position	Sep. 30, 2016 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 494,309	$ 494,309	$ 527,948
Unrealized gains	3,424	3,424	1,581
Unrealized losses	2,322	2,322	4,479
Estimated fair value	495,411	495,411	525,050
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	117,371	117,371	286,063
12 months or longer	44,218	44,218	35,209
Total	161,589	161,589	321,272
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(1,033)	(1,033)	(3,290)
12 months or longer	(1,289)	(1,289)	(1,189)
Total	$ (2,322)	$ (2,322)	$ (4,479)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	32	32	172
12 months or longer | Position	25	25	4
Total | Position	57	57	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	1,200,000	1,200,000	$ 1,500,000
Amortized costs of securities held to maturity pledged as collateral for repurchase agreements	242,600	242,600	235,600
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Within one year	28,489	28,489
From one to five years	38,321	38,321
From five to ten years	144,621	144,621
After ten years	282,878	282,878
Amortized cost	494,309	494,309	527,948
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Within one year	28,493	28,493
From one to five years	38,702	38,702
From five to ten years	145,547	145,547
After ten years	282,669	282,669
Fair value	495,411	495,411	525,050
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	51,166	51,166	95,492
12 months or longer	0	0	0
Total	51,166	51,166	95,492
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(685)	(685)	(1,176)
12 months or longer	0	0	0
Total	$ (685)	$ (685)	$ (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	$ 99,502	$ 99,502	$ 96,668
Unrealized gains	239	239	0
Unrealized losses	685	685	1,176
Estimated fair value	99,056	99,056	95,492
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	99,502	99,502	96,668
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	99,056	99,056	95,492
Mortgage-Backed, U.S. Government Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	447	447	533
Unrealized gains	67	67	87
Unrealized losses	0	0	0
Estimated fair value	514	514	620
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	447	447	533
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	514	514	620
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	49,394	49,394	108,587
12 months or longer	31,627	31,627	35,209
Total	81,021	81,021	143,796
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(160)	(160)	(319)
12 months or longer	(943)	(943)	(1,189)
Total	$ (1,103)	$ (1,103)	$ (1,508)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	7	7	12
12 months or longer | Position	4	4	4
Total | Position	11	11	16
Collateralized Mortgage Obligations, Government-Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 195,388	$ 195,388	$ 225,213
Unrealized gains	1,135	1,135	1,060
Unrealized losses	1,103	1,103	1,508
Estimated fair value	195,420	195,420	224,765
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	195,388	195,388	225,213
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	195,420	195,420	224,765
State & Municipal [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	198,972	198,972	205,534
Unrealized gains	1,983	1,983	434
Unrealized losses	534	534	1,795
Estimated fair value	200,421	200,421	204,173
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	16,811	16,811	81,984
12 months or longer	12,591	12,591	0
Total	29,402	29,402	81,984
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(188)	(188)	(1,795)
12 months or longer	(346)	(346)	0
Total	$ (534)	$ (534)	$ (1,795)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	21	21	155
12 months or longer | Position	21	21	0
Total | Position	42	42	155
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	$ 198,972	$ 198,972	$ 205,534
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	$ 200,421	$ 200,421	$ 204,173