Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available for sale:
Total	$ 1,078,515	$ 786,783
Unrealized Gain	935	6,161
Unrealized Loss	(24,573)	(5,515)
Estimated Fair Value	1,054,877	787,429
Held to maturity:
Amortized Cost	46,385	18,291
Unrealized Gain	22	0
Unrealized Loss	(1,269)	(209)
Estimated Fair Value	45,138	18,082
Total investment securities
Amortized Cost	1,124,900	805,074
Unrealized Gain	957	6,161
Unrealized Loss	(25,842)	(5,724)
Estimated Fair Value	1,100,015	805,511
U.S. Treasury
Available for sale:
Total	59,659
Unrealized Gain	13
Unrealized Loss	(329)
Estimated Fair Value	59,343
Agency
Available for sale:
Total	113,628	47,051
Unrealized Gain	20	236
Unrealized Loss	(877)	(78)
Estimated Fair Value	112,771	47,209
Corporate
Available for sale:
Total	102,761	78,155
Unrealized Gain	235	1,585
Unrealized Loss	(1,402)	(194)
Estimated Fair Value	101,594	79,546
Municipal bonds
Available for sale:
Total	234,910	228,929
Unrealized Gain	584	3,942
Unrealized Loss	(4,293)	(743)
Estimated Fair Value	231,201	232,128
Collateralized mortgage obligation: residential
Available for sale:
Total	25,897	33,984
Unrealized Gain	50	132
Unrealized Loss	(741)	(335)
Estimated Fair Value	25,206	33,781
Mortgage-backed securities: residential
Available for sale:
Total	541,660	398,664
Unrealized Gain	33	266
Unrealized Loss	(16,931)	(4,165)
Estimated Fair Value	524,762	394,765
Held to maturity:
Amortized Cost	45,287	17,153
Unrealized Gain	22	0
Unrealized Loss	(1,269)	(209)
Estimated Fair Value	44,040	16,944
Other
Held to maturity:
Amortized Cost	1,098	1,138
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	$ 1,098	$ 1,138