Securities, Available for Sale (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Position	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Position	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Position
Debt securities, available-for-sale [Abstract]
Amortized cost	$ 1,227,513		$ 1,227,513
Unrealized gains	926		926
Unrealized losses	35,500		35,500
Estimated fair value	1,192,939		1,192,939
Total available-for-sale securities [Abstract]
Amortized cost					$ 1,266,165
Unrealized gains					5,893
Unrealized losses					16,133
Estimated fair value	1,192,939		1,192,939		1,255,925
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	727,989		727,989		633,766
12 months or longer	406,015		406,015		386,818
Total	1,134,004		1,134,004		1,020,584
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(18,424)		(18,424)		(5,481)
12 months or longer	(17,076)		(17,076)		(10,652)
Total	$ (35,500)		$ (35,500)		$ (16,133)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	174		174		134
12 months or longer | Position	86		86		79
Total | Position	260		260		213
OTTI loss realized on equity investment	$ 0	$ 0	$ 0	$ 1,300
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Within one year	79,927		79,927
From one to five years	78,012		78,012
From five to ten years	186,454		186,454
After ten years	883,120		883,120
Amortized cost	1,227,513		1,227,513
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Within one year	79,698		79,698
From one to five years	76,566		76,566
From five to ten years	182,532		182,532
After ten years	854,143		854,143
Fair value	1,192,939		1,192,939
Federal Agency [Member]
Debt securities, available-for-sale [Abstract]
Amortized cost	119,964		119,964		$ 109,862
Unrealized gains	0		0		0
Unrealized losses	1,394		1,394		963
Estimated fair value	118,570		118,570		108,899
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	39,457		39,457		64,653
12 months or longer	79,113		79,113		44,246
Total	118,570		118,570		108,899
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(526)		(526)		(242)
12 months or longer	(868)		(868)		(721)
Total	$ (1,394)		$ (1,394)		$ (963)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	4		4		5
12 months or longer | Position	6		6		4
Total | Position	10		10		9
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	$ 119,964		$ 119,964		$ 109,862
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	118,570		118,570		108,899
State & Municipal [Member]
Debt securities, available-for-sale [Abstract]
Amortized cost	36,676		36,676		42,171
Unrealized gains	28		28		62
Unrealized losses	302		302		277
Estimated fair value	36,402		36,402		41,956
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	23,273		23,273		23,566
12 months or longer	5,479		5,479		5,994
Total	28,752		28,752		29,560
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(232)		(232)		(200)
12 months or longer	(70)		(70)		(77)
Total	$ (302)		$ (302)		$ (277)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	37		37		39
12 months or longer | Position	8		8		8
Total | Position	45		45		47
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	$ 36,676		$ 36,676		$ 42,171
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	36,402		36,402		41,956
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	432,173		432,173		317,630
12 months or longer	60,657		60,657		58,316
Total	492,830		492,830		375,946
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(11,590)		(11,590)		(2,381)
12 months or longer	(2,832)		(2,832)		(1,187)
Total	$ (14,422)		$ (14,422)		$ (3,568)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	93		93		55
12 months or longer | Position	26		26		24
Total | Position	119		119		79
Mortgage-Backed, Government Sponsored Enterprises [Member]
Debt securities, available-for-sale [Abstract]
Amortized cost	$ 503,303		$ 503,303		$ 530,392
Unrealized gains	307		307		1,406
Unrealized losses	13,579		13,579		3,345
Estimated fair value	490,031		490,031		528,453
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	503,303		503,303		530,392
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	490,031		490,031		528,453
Mortgage-Backed, U.S. Government Agency Securities [Member]
Debt securities, available-for-sale [Abstract]
Amortized cost	30,232		30,232		26,363
Unrealized gains	225		225		334
Unrealized losses	843		843		223
Estimated fair value	29,614		29,614		26,474
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	30,232		30,232		26,363
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	29,614		29,614		26,474
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	233,086		233,086		227,917
12 months or longer	260,766		260,766		275,303
Total	493,852		493,852		503,220
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(6,076)		(6,076)		(2,658)
12 months or longer	(13,306)		(13,306)		(8,521)
Total	$ (19,382)		$ (19,382)		$ (11,179)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	40		40		35
12 months or longer | Position	46		46		42
Total | Position	86		86		77
Collateralized Mortgage Obligations, Government-Sponsored Enterprises [Member]
Debt securities, available-for-sale [Abstract]
Amortized cost	$ 486,029		$ 486,029		$ 496,033
Unrealized gains	234		234		254
Unrealized losses	17,702		17,702		10,114
Estimated fair value	468,561		468,561		486,173
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	486,029		486,029		496,033
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	468,561		468,561		486,173
Collateralized Mortgage Obligations, U.S. Government Agency Securities [Member]
Debt securities, available-for-sale [Abstract]
Amortized cost	51,309		51,309		50,721
Unrealized gains	132		132		165
Unrealized losses	1,680		1,680		1,065
Estimated fair value	49,761		49,761		49,821
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	51,309		51,309		50,721
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Fair value	$ 49,761		$ 49,761		49,821
Equity Securities [Member]
Equity securities, available-for-sale [Abstract]
Amortized cost					10,623
Unrealized gains					3,672
Unrealized losses					146
Estimated fair value					14,149
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months					0
12 months or longer					2,959
Total					2,959
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months					0
12 months or longer					(146)
Total					$ (146)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position					0
12 months or longer | Position					1
Total | Position					1