Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available for sale:
Total	$ 1,226,954	$ 1,111,168
Unrealized Gain	34,005	6,263
Unrealized Loss	(2,580)	(14,209)
Estimated Fair Value	1,258,379	1,103,222
Held to maturity:
Amortized Cost	42,997	45,210
Unrealized Gain	790	148
Unrealized Loss	(128)	(686)
Estimated Fair Value	43,659	44,672
Total investment securities
Amortized Cost	1,269,951	1,156,378
Unrealized Gain	34,795	6,411
Unrealized Loss	(2,708)	(14,895)
Estimated Fair Value	1,302,038	1,147,894
U.S. Treasury
Available for sale:
Total	59,745	59,688
Unrealized Gain	3,807	1,224
Unrealized Loss	0	0
Estimated Fair Value	63,552	60,912
Agency
Available for sale:
Total	189,832	128,958
Unrealized Gain	9,706	1,631
Unrealized Loss	(305)	(519)
Estimated Fair Value	199,233	130,070
Corporate
Available for sale:
Total	122,755	104,158
Unrealized Gain	1,447	291
Unrealized Loss	(83)	(906)
Estimated Fair Value	124,119	103,543
Municipal bonds
Available for sale:
Total	278,962	238,914
Unrealized Gain	9,573	1,941
Unrealized Loss	(129)	(2,225)
Estimated Fair Value	288,406	238,630
Collateralized mortgage obligation: residential
Available for sale:
Total	19,197	24,699
Unrealized Gain	93	64
Unrealized Loss	(54)	(425)
Estimated Fair Value	19,236	24,338
Mortgage-backed securities: residential
Available for sale:
Total	556,463	554,751
Unrealized Gain	9,379	1,112
Unrealized Loss	(2,009)	(10,134)
Estimated Fair Value	563,833	545,729
Held to maturity:
Amortized Cost	41,225	43,381
Unrealized Gain	790	148
Unrealized Loss	(128)	(686)
Estimated Fair Value	41,887	42,843
Other
Held to maturity:
Amortized Cost	1,772	1,829
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	$ 1,772	$ 1,829