Investment Securities - Amortized Cost, Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,816,527	$ 1,511,034
Gross Unrealized Gains	1,683	5,029
Gross Unrealized Losses	(69,209)	(22,158)
Fair Value	1,749,001	1,493,905
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,980	50,997
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2,617)	(1,355)
Fair Value	48,363	49,642
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,618	42,384
Gross Unrealized Gains	67	158
Gross Unrealized Losses	(1,172)	(37)
Fair Value	36,513	42,505
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	106,881	38,435
Gross Unrealized Gains	41	158
Gross Unrealized Losses	(3,344)	(374)
Fair Value	103,578	38,219
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	103,631	74,752
Gross Unrealized Gains	137	477
Gross Unrealized Losses	(2,109)	(564)
Fair Value	101,659	74,665
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	210,512	113,187
Gross Unrealized Gains	178	635
Gross Unrealized Losses	(5,453)	(938)
Fair Value	205,237	112,884
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,470,565	1,239,534
Gross Unrealized Gains	1,304	2,423
Gross Unrealized Losses	(59,020)	(18,662)
Fair Value	1,412,849	1,223,295
Trust preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,789	3,726
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(199)	(512)
Fair Value	1,590	3,214
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	45,063	50,701
Gross Unrealized Gains	134	623
Gross Unrealized Losses	(748)	(160)
Fair Value	$ 44,449	51,164
Equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		10,505
Gross Unrealized Gains		1,190
Gross Unrealized Losses		(494)
Fair Value		$ 11,201

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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).

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