Investment Securities - Continuous Unrealized Loss on Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	$ (7,855)	$ (2,434)
Less than 12 months, Fair value	318,416	265,576
Twelve months or more, Gross unrealized losses	(25,143)	(11,571)
Twelve months or more, Fair value	465,519	414,529
Total, Gross unrealized losses	(32,998)	(14,005)
Total, Fair value	783,935	680,105
HELD TO MATURITY
Less than 12 months, Gross unrealized losses	(709)	(177)
Less than 12 months, Fair value	36,753	24,186
Twelve months or more, Gross unrealized losses	(933)	(180)
Twelve months or more, Fair value	22,240	13,917
Total, Gross unrealized losses	(1,642)	(357)
Total, Fair value	58,993	38,103
Residential mortgage-backed securities [Member]
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	(151)	(182)
Less than 12 months, Fair value	4,665	18,020
Twelve months or more, Gross unrealized losses	(6,078)	(3,386)
Twelve months or more, Fair value	105,148	110,878
Total, Gross unrealized losses	(6,229)	(3,568)
Total, Fair value	109,813	128,898
HELD TO MATURITY
Less than 12 months, Gross unrealized losses	(135)	(13)
Less than 12 months, Fair value	4,591	2,662
Twelve months or more, Gross unrealized losses	(244)	(86)
Twelve months or more, Fair value	4,066	4,452
Total, Gross unrealized losses	(379)	(99)
Total, Fair value	8,657	7,114
Commercial mortgage-backed securities [Member]
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	(517)	(113)
Less than 12 months, Fair value	21,504	15,265
Twelve months or more, Gross unrealized losses	(535)	(225)
Twelve months or more, Fair value	12,795	6,748
Total, Gross unrealized losses	(1,052)	(338)
Total, Fair value	34,299	22,013
HELD TO MATURITY
Less than 12 months, Gross unrealized losses	(168)	(161)
Less than 12 months, Fair value	7,740	15,900
Twelve months or more, Gross unrealized losses	(385)	0
Twelve months or more, Fair value	9,095	0
Total, Gross unrealized losses	(553)	(161)
Total, Fair value	16,835	15,900
Municipal bonds [Member]
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	(4,309)	(760)
Less than 12 months, Fair value	175,456	105,415
Twelve months or more, Gross unrealized losses	(7,852)	(2,883)
Twelve months or more, Fair value	162,506	134,103
Total, Gross unrealized losses	(12,161)	(3,643)
Total, Fair value	337,962	239,518
HELD TO MATURITY
Less than 12 months, Gross unrealized losses	(297)	(3)
Less than 12 months, Fair value	11,468	2,185
Twelve months or more, Gross unrealized losses	(304)	(94)
Twelve months or more, Fair value	9,079	9,465
Total, Gross unrealized losses	(601)	(97)
Total, Fair value	20,547	11,650
Collateralized mortgage obligations [Member]
HELD TO MATURITY
Less than 12 months, Gross unrealized losses	(109)	0
Less than 12 months, Fair value	12,954	3,439
Twelve months or more, Gross unrealized losses	0	0
Twelve months or more, Fair value	0	0
Total, Gross unrealized losses	(109)	0
Total, Fair value	12,954	3,439
Residential collateralized mortgage obligations [Member]
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	(1,228)	(612)
Less than 12 months, Fair value	49,619	53,721
Twelve months or more, Gross unrealized losses	(6,869)	(3,469)
Twelve months or more, Fair value	109,677	104,555
Total, Gross unrealized losses	(8,097)	(4,081)
Total, Fair value	159,296	158,276
Commercial collateralized mortgage obligations [Member]
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	(1,488)	(538)
Less than 12 months, Fair value	58,535	57,236
Twelve months or more, Gross unrealized losses	(1,828)	(903)
Twelve months or more, Fair value	44,308	35,225
Total, Gross unrealized losses	(3,316)	(1,441)
Total, Fair value	102,843	92,461
Corporate debt securities [Member]
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	(162)	(15)
Less than 12 months, Fair value	8,637	5,272
Twelve months or more, Gross unrealized losses	(889)	(461)
Twelve months or more, Fair value	12,394	13,365
Total, Gross unrealized losses	(1,051)	(476)
Total, Fair value	21,031	18,637
US Treasury securities [Member]
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	0	(3)
Less than 12 months, Fair value	0	997
Twelve months or more, Gross unrealized losses	(537)	(244)
Twelve months or more, Fair value	9,374	9,655
Total, Gross unrealized losses	(537)	(247)
Total, Fair value	9,374	10,652
Agency debentures [Member]
AVAILABLE FOR SALE
Less than 12 months, Gross unrealized losses	0	(211)
Less than 12 months, Fair value	0	9,650
Twelve months or more, Gross unrealized losses	(555)	0
Twelve months or more, Fair value	9,317	0
Total, Gross unrealized losses	(555)	(211)
Total, Fair value	$ 9,317	$ 9,650

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

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