Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	$ 899,537,000		$ 899,537,000		$ 1,063,374,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(6,382,000)		(6,382,000)		(19,560,000)
Available-for-Sale, 12 Months or More, Fair Value	161,481,000		161,481,000		15,849,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(1,737,000)		(1,737,000)		(500,000)
Available-for-Sale, Total Fair Value	1,061,018,000		1,061,018,000		1,079,223,000
Available-for-Sale, Total Gross Unrealized Losses	(8,119,000)		(8,119,000)		(20,060,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	77,857,000		77,857,000		203,424,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(8,930,000)		(8,930,000)		(8,879,000)
Held-to-Maturity, 12 Months or More, Fair Value	28,071,000		28,071,000		22,817,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(1,417,000)		(1,417,000)		(1,285,000)
Held-to-Maturity, Total Fair Value	105,928,000		105,928,000		226,241,000
Held-to-Maturity, Total Gross Unrealized Losses	$ (10,347,000)		$ (10,347,000)		$ (10,164,000)
Investment securities in an unrealized loss position (securities) | security	358		358		396
Impairment loss	$ 0	$ 0	$ 0	$ 0
U.S. Treasury notes
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	2,201,000		2,201,000		$ 598,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(2,000)		(2,000)		(1,000)
Available-for-Sale, 12 Months or More, Fair Value	0		0		0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	0		0		0
Available-for-Sale, Total Fair Value	2,201,000		2,201,000		598,000
Available-for-Sale, Total Gross Unrealized Losses	(2,000)		(2,000)		(1,000)
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	303,060,000		303,060,000		316,511,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(2,458,000)		(2,458,000)		(6,457,000)
Available-for-Sale, 12 Months or More, Fair Value	86,718,000		86,718,000		0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(751,000)		(751,000)		0
Available-for-Sale, Total Fair Value	389,778,000		389,778,000		316,511,000
Available-for-Sale, Total Gross Unrealized Losses	(3,209,000)		(3,209,000)		(6,457,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	9,972,000		9,972,000		19,575,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(19,000)		(19,000)		(162,000)
Held-to-Maturity, 12 Months or More, Fair Value	0		0		0
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	0		0		0
Held-to-Maturity, Total Fair Value	9,972,000		9,972,000		19,575,000
Held-to-Maturity, Total Gross Unrealized Losses	(19,000)		(19,000)		(162,000)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	556,771,000		556,771,000		746,265,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(3,731,000)		(3,731,000)		(13,102,000)
Available-for-Sale, 12 Months or More, Fair Value	74,732,000		74,732,000		15,801,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(985,000)		(985,000)		(499,000)
Available-for-Sale, Total Fair Value	631,503,000		631,503,000		762,066,000
Available-for-Sale, Total Gross Unrealized Losses	(4,716,000)		(4,716,000)		(13,601,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	38,712,000		38,712,000		108,857,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(8,814,000)		(8,814,000)		(7,973,000)
Held-to-Maturity, 12 Months or More, Fair Value	18,846,000		18,846,000		19,986,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(1,358,000)		(1,358,000)		(1,276,000)
Held-to-Maturity, Total Fair Value	57,558,000		57,558,000		128,843,000
Held-to-Maturity, Total Gross Unrealized Losses	(10,172,000)		(10,172,000)		(9,249,000)
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	30,518,000		30,518,000		0
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(189,000)		(189,000)		0
Available-for-Sale, 12 Months or More, Fair Value	31,000		31,000		48,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(1,000)		(1,000)		(1,000)
Available-for-Sale, Total Fair Value	30,549,000		30,549,000		48,000
Available-for-Sale, Total Gross Unrealized Losses	(190,000)		(190,000)		(1,000)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	25,207,000		25,207,000		42,518,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(97,000)		(97,000)		(533,000)
Held-to-Maturity, 12 Months or More, Fair Value	4,208,000		4,208,000		2,831,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(26,000)		(26,000)		(9,000)
Held-to-Maturity, Total Fair Value	29,415,000		29,415,000		45,349,000
Held-to-Maturity, Total Gross Unrealized Losses	(123,000)		(123,000)		(542,000)
Corporate securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	6,987,000		6,987,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(2,000)		(2,000)
Available-for-Sale, 12 Months or More, Fair Value	0		0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	0		0
Available-for-Sale, Total Fair Value	6,987,000		6,987,000
Available-for-Sale, Total Gross Unrealized Losses	(2,000)		(2,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	3,966,000		3,966,000		32,474,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	0		0		(211,000)
Held-to-Maturity, 12 Months or More, Fair Value	5,017,000		5,017,000		0
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(33,000)		(33,000)		0
Held-to-Maturity, Total Fair Value	8,983,000		8,983,000		32,474,000
Held-to-Maturity, Total Gross Unrealized Losses	$ (33,000)		$ (33,000)		$ (211,000)