Investment Securities - Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Held-to-Maturity
Held-to-maturity debt securities, less than 12 months, estimated fair value	$ 40,141	$ 57,731
Held-to-maturity debt securities, less than 12 months, gross unrealized losses	(210)	(251)
Held-to-maturity debt securities, 12 months or more, estimated fair value	79,014	43,611
Held-to-maturity debt securities,12 months or more, gross unrealized losses	(1,205)	(486)
Held-to-maturity debt securities, estimated fair value	119,155	101,342
Held-to-maturity debt securities, gross unrealized losses	(1,415)	(737)
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	319,262	627,807
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(2,786)	(5,639)
Available-for-sale debt securities, 12 months or more, estimated fair value	1,278,434	710,574
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(39,122)	(19,268)
Available-for-sale debt securities, estimated fair value	1,597,696	1,338,381
Available-for-sale debt securities, gross unrealized losses	(41,908)	(24,907)
U.S. Government agencies
Held-to-Maturity
Held-to-maturity debt securities, less than 12 months, estimated fair value	992	11,961
Held-to-maturity debt securities, less than 12 months, gross unrealized losses	(2)	(6)
Held-to-maturity debt securities, 12 months or more, estimated fair value	15,948	32,778
Held-to-maturity debt securities,12 months or more, gross unrealized losses	(47)	(222)
Held-to-maturity debt securities, estimated fair value	16,940	44,739
Held-to-maturity debt securities, gross unrealized losses	(49)	(228)
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	26,562	91,988
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(221)	(921)
Available-for-sale debt securities, 12 months or more, estimated fair value	108,636	25,742
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(3,519)	(1,030)
Available-for-sale debt securities, estimated fair value	135,198	117,730
Available-for-sale debt securities, gross unrealized losses	(3,740)	(1,951)
Mortgage-backed securities
Held-to-Maturity
Held-to-maturity debt securities, less than 12 months, estimated fair value	0	5,471
Held-to-maturity debt securities, less than 12 months, gross unrealized losses	0	(47)
Held-to-maturity debt securities, 12 months or more, estimated fair value	12,177	8,946
Held-to-maturity debt securities,12 months or more, gross unrealized losses	(412)	(240)
Held-to-maturity debt securities, estimated fair value	12,177	14,417
Held-to-maturity debt securities, gross unrealized losses	(412)	(287)
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	163,560	510,770
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(1,146)	(4,516)
Available-for-sale debt securities, 12 months or more, estimated fair value	1,037,679	609,329
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(31,538)	(16,430)
Available-for-sale debt securities, estimated fair value	1,201,239	1,120,099
Available-for-sale debt securities, gross unrealized losses	(32,684)	(20,946)
State and political subdivisions
Held-to-Maturity
Held-to-maturity debt securities, less than 12 months, estimated fair value	39,149	40,299
Held-to-maturity debt securities, less than 12 months, gross unrealized losses	(208)	(198)
Held-to-maturity debt securities, 12 months or more, estimated fair value	50,889	1,887
Held-to-maturity debt securities,12 months or more, gross unrealized losses	(746)	(24)
Held-to-maturity debt securities, estimated fair value	90,038	42,186
Held-to-maturity debt securities, gross unrealized losses	(954)	(222)
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	129,075	25,049
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(1,406)	(202)
Available-for-sale debt securities, 12 months or more, estimated fair value	132,020	75,404
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(4,064)	(1,807)
Available-for-sale debt securities, estimated fair value	261,095	100,453
Available-for-sale debt securities, gross unrealized losses	(5,470)	(2,009)
Other securities
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	65	0
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(13)	0
Available-for-sale debt securities, 12 months or more, estimated fair value	99	99
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(1)	(1)
Available-for-sale debt securities, estimated fair value	164	99
Available-for-sale debt securities, gross unrealized losses	$ (14)	$ (1)

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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 amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.

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