Investment Securities - Unrealized Losses and Fair Value of Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less than Twelve Months	$ 744,684	$ 365,182
More than Twelve Months, Fair Value	746,275	663,238
Total	1,490,959	1,028,420
Unrealized Losses
Less than Twelve Months	(16,110)	(2,702)
More than Twelve Months	(49,688)	(27,788)
Total	(65,798)	(30,490)
U.S. Treasury securities
Fair Value
Less than Twelve Months	34,660
More than Twelve Months, Fair Value	0
Total	34,660
Unrealized Losses
Less than Twelve Months	(141)
More than Twelve Months	0
Total	(141)
U.S. government agency securities
Fair Value
Less than Twelve Months	8,541
More than Twelve Months, Fair Value	0
Total	8,541
Unrealized Losses
Less than Twelve Months	(305)
More than Twelve Months	0
Total	(305)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	129,321	6,342
More than Twelve Months, Fair Value	48,862	53,034
Total	178,183	59,376
Unrealized Losses
Less than Twelve Months	(3,414)	(50)
More than Twelve Months	(3,556)	(1,603)
Total	(6,970)	(1,653)
Municipal bonds
Fair Value
Less than Twelve Months	89,065	4,644
More than Twelve Months, Fair Value	528	561
Total	89,593	5,205
Unrealized Losses
Less than Twelve Months	(2,208)	(25)
More than Twelve Months	(37)	(3)
Total	(2,245)	(28)
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	218,386	10,986
More than Twelve Months, Fair Value	49,390	53,595
Total	267,776	64,581
Unrealized Losses
Less than Twelve Months	(5,622)	(75)
More than Twelve Months	(3,593)	(1,606)
Total	(9,215)	(1,681)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	467,711	344,216
More than Twelve Months, Fair Value	662,345	570,969
Total	1,130,056	915,185
Unrealized Losses
Less than Twelve Months	(9,928)	(2,357)
More than Twelve Months	(39,661)	(16,386)
Total	(49,589)	(18,743)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months, Fair Value	29,761	38,674
Total	29,761	38,674
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(6,213)	(9,796)
Total	(6,213)	(9,796)
Corporate and other debt securities
Fair Value
Less than Twelve Months	15,386	9,980
More than Twelve Months, Fair Value	4,779	0
Total	20,165	9,980
Unrealized Losses
Less than Twelve Months	(114)	(270)
More than Twelve Months	(221)	0
Total	$ (335)	$ (270)

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

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-Publisher FASB
-Name Accounting Standards Codification
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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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Fair value of investment in debt security measured at amortized cost (held-to-maturity), 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 less than 12 months.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

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