Investment Securities (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value
Less than 12 Months	$ 656,335		$ 497,752
12 months or Longer	183,611		184,736
Total fair value	839,946		682,488
Unrealized Losses
Less than 12 Months	(8,728)		(2,841)
12 months or Longer	(7,009)		(4,670)
Total unrealized losses	(15,737)		(7,511)
Gross realized gains and losses
Gross realized gains	2	$ 1,285
Gross realized losses	(347)	0
OTTI losses	0	0
Net realized gains (losses)	(345)	$ 1,285
Investment securities pledged as collateral	262,740		289,050
U.S. Treasury and Federal agencies securities
Fair Value
Less than 12 Months	399,359		311,865
12 months or Longer	88,465		89,617
Total fair value	487,824		401,482
Unrealized Losses
Less than 12 Months	(4,388)		(1,161)
12 months or Longer	(3,361)		(2,285)
Total unrealized losses	(7,749)		(3,446)
U.S. States and political subdivisions securities
Fair Value
Less than 12 Months	46,927		34,971
12 months or Longer	18,067		24,909
Total fair value	64,994		59,880
Unrealized Losses
Less than 12 Months	(484)		(287)
12 months or Longer	(592)		(621)
Total unrealized losses	(1,076)		(908)
Mortgage-backed securities - Federal agencies
Fair Value
Less than 12 Months	175,903		137,169
12 months or Longer	67,169		60,162
Total fair value	243,072		197,331
Unrealized Losses
Less than 12 Months	(3,672)		(1,336)
12 months or Longer	(2,712)		(1,537)
Total unrealized losses	(6,384)		(2,873)
Corporate debt securities
Fair Value
Less than 12 Months	34,146		13,747
12 months or Longer	9,910		10,048
Total fair value	44,056		23,795
Unrealized Losses
Less than 12 Months	(184)		(57)
12 months or Longer	(344)		(227)
Total unrealized losses	(528)		(284)
Foreign government and other securities
Fair Value
Less than 12 Months	0		0
12 months or Longer	0		0
Total fair value	0		0
Unrealized Losses
Less than 12 Months	0		0
12 months or Longer	0		0
Total unrealized losses	$ 0		$ 0