Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available for sale:
Total	$ 1,164,340	$ 1,111,168
Unrealized Gain	14,571	6,263
Unrealized Loss	(7,501)	(14,209)
Estimated Fair Value	1,171,410	1,103,222
Held to maturity:
Amortized Cost	43,894	45,210
Unrealized Gain	466	148
Unrealized Loss	(380)	(686)
Estimated Fair Value	43,980	44,672
Total investment securities
Amortized Cost	1,208,234	1,156,378
Unrealized Gain	15,037	6,411
Unrealized Loss	(7,881)	(14,895)
Estimated Fair Value	1,215,390	1,147,894
U.S. Treasury
Available for sale:
Total	59,716	59,688
Unrealized Gain	2,185	1,224
Unrealized Loss	0	0
Estimated Fair Value	61,901	60,912
Agency
Available for sale:
Total	161,945	128,958
Unrealized Gain	3,987	1,631
Unrealized Loss	(444)	(519)
Estimated Fair Value	165,488	130,070
Corporate
Available for sale:
Total	138,128	104,158
Unrealized Gain	538	291
Unrealized Loss	(811)	(906)
Estimated Fair Value	137,855	103,543
Municipal bonds
Available for sale:
Total	213,561	238,914
Unrealized Gain	4,122	1,941
Unrealized Loss	(667)	(2,225)
Estimated Fair Value	217,016	238,630
Collateralized mortgage obligation: residential
Available for sale:
Total	23,642	24,699
Unrealized Gain	70	64
Unrealized Loss	(261)	(425)
Estimated Fair Value	23,451	24,338
Mortgage-backed securities: residential
Available for sale:
Total	567,348	554,751
Unrealized Gain	3,669	1,112
Unrealized Loss	(5,318)	(10,134)
Estimated Fair Value	565,699	545,729
Held to maturity:
Amortized Cost	42,083	43,381
Unrealized Gain	466	148
Unrealized Loss	(380)	(686)
Estimated Fair Value	42,169	42,843
Other
Held to maturity:
Amortized Cost	1,811	1,829
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	$ 1,811	$ 1,829