Securities, Available for Sale (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Position	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Position
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 1,295,087		$ 1,266,165
Unrealized gains	1,215		5,893
Unrealized losses	30,390		16,133
Estimated fair value	1,265,912		1,255,925
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	790,421		633,766
12 months or longer	383,843		386,818
Total	1,174,264		1,020,584
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(15,642)		(5,481)
12 months or longer	(14,748)		(10,652)
Total	$ (30,390)		$ (16,133)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	178		134
12 months or longer | Position	81		79
Total | Position	259		213
OTTI loss realized on equity investment	$ 0	$ 1,300
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Within one year	99,647
From one to five years	82,353
From five to ten years	175,040
After ten years	938,047
Amortized cost	1,295,087
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Within one year	99,288
From one to five years	81,136
From five to ten years	172,862
After ten years	912,626
Fair value	1,265,912
Federal Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	139,775		$ 109,862
Unrealized gains	0		0
Unrealized losses	1,316		963
Estimated fair value	138,459		108,899
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	74,268		64,653
12 months or longer	64,191		44,246
Total	138,459		108,899
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(459)		(242)
12 months or longer	(857)		(721)
Total	$ (1,316)		$ (963)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	6		5
12 months or longer | Position	6		4
Total | Position	12		9
State & Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 39,700		$ 42,171
Unrealized gains	31		62
Unrealized losses	345		277
Estimated fair value	39,386		41,956
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	25,927		23,566
12 months or longer	4,979		5,994
Total	30,906		29,560
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(271)		(200)
12 months or longer	(74)		(77)
Total	$ (345)		$ (277)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	44		39
12 months or longer | Position	7		8
Total | Position	51		47
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 440,714		$ 317,630
12 months or longer	55,185		58,316
Total	495,899		375,946
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(9,582)		(2,381)
12 months or longer	(2,118)		(1,187)
Total	$ (11,700)		$ (3,568)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	90		55
12 months or longer | Position	25		24
Total | Position	115		79
Mortgage-Backed, Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 524,468		$ 530,392
Unrealized gains	523		1,406
Unrealized losses	11,155		3,345
Estimated fair value	513,836		528,453
Mortgage-Backed, U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	30,473		26,363
Unrealized gains	267		334
Unrealized losses	545		223
Estimated fair value	30,195		26,474
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	249,512		227,917
12 months or longer	259,488		275,303
Total	509,000		503,220
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(5,330)		(2,658)
12 months or longer	(11,699)		(8,521)
Total	$ (17,029)		$ (11,179)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	38		35
12 months or longer | Position	43		42
Total | Position	81		77
Collateralized Mortgage Obligations, Government-Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 511,727		$ 496,033
Unrealized gains	243		254
Unrealized losses	15,541		10,114
Estimated fair value	496,429		486,173
Collateralized Mortgage Obligations, U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	48,944		50,721
Unrealized gains	151		165
Unrealized losses	1,488		1,065
Estimated fair value	$ 47,607		49,821
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
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