Securities, Held to Maturity (Details) $ in Thousands	Mar. 31, 2018 USD ($) Position	Dec. 31, 2017 USD ($) Position
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 487,126	$ 484,073
Unrealized gains	339	1,805
Unrealized losses	10,056	4,007
Estimated fair value	477,409	481,871
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	281,188	156,340
12 months or longer	82,506	87,037
Total	363,694	243,377
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(5,507)	(1,336)
12 months or longer	(4,549)	(2,671)
Total	$ (10,056)	$ (4,007)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	145	59
12 months or longer | Position	32	32
Total | Position	177	91
Amortized costs of securities held to maturity pledged to secure public deposits	$ 1,600,000	$ 1,500,000
Amortized costs of securities held to maturity pledged as collateral for repurchase agreements	218,300	231,300
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Within one year	37,885
From one to five years	43,089
From five to ten years	208,715
After ten years	197,437
Amortized cost	487,126	484,073
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Within one year	37,885
From one to five years	43,159
From five to ten years	204,710
After ten years	191,655
Fair value	477,409	481,871
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	59,070	15,477
12 months or longer	31,950	33,703
Total	91,020	49,180
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(1,344)	(140)
12 months or longer	(1,405)	(670)
Total	$ (2,749)	$ (810)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	4	2
12 months or longer | Position	2	2
Total | Position	6	4
Mortgage-Backed, Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 93,769	$ 96,357
Unrealized gains	0	85
Unrealized losses	2,749	810
Estimated fair value	91,020	95,632
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	93,769	96,357
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	91,020	95,632
Mortgage-Backed, U.S. Government Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	389	418
Unrealized gains	47	57
Unrealized losses	0	0
Estimated fair value	436	475
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	389	418
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	436	475
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	147,877	118,476
12 months or longer	35,379	37,614
Total	183,256	156,090
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(2,864)	(1,064)
12 months or longer	(2,140)	(1,513)
Total	$ (5,004)	$ (2,577)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	23	17
12 months or longer | Position	6	6
Total | Position	29	23
Collateralized Mortgage Obligations, Government-Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 188,260	$ 186,327
Unrealized gains	0	224
Unrealized losses	5,004	2,577
Estimated fair value	183,256	183,974
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	188,260	186,327
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	183,256	183,974
State & Municipal [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	204,708	200,971
Unrealized gains	292	1,439
Unrealized losses	2,303	620
Estimated fair value	202,697	201,790
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	74,241	22,387
12 months or longer	15,177	15,720
Total	89,418	38,107
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(1,299)	(132)
12 months or longer	(1,004)	(488)
Total	$ (2,303)	$ (620)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	118	40
12 months or longer | Position	24	24
Total | Position	142	64
Held-to-maturity Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	$ 204,708	$ 200,971
Held-to-maturity Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	$ 202,697	$ 201,790