Fair Value of Financial Instruments - Hierarchy Table - Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial assets
Investment securities available-for-sale, at fair value	$ 1,054,877	$ 787,429
U.S. Treasury
Financial assets
Investment securities available-for-sale, at fair value	59,343
Agency
Financial assets
Investment securities available-for-sale, at fair value	112,771	47,209
Corporate
Financial assets
Investment securities available-for-sale, at fair value	101,594	79,546
Municipal bonds
Financial assets
Investment securities available-for-sale, at fair value	231,201	232,128
Collateralized mortgage obligation: residential
Financial assets
Investment securities available-for-sale, at fair value	25,206	33,781
Mortgage-backed securities: residential
Financial assets
Investment securities available-for-sale, at fair value	524,762	394,765
Level 1
Financial assets
Investment securities available-for-sale, at fair value	0	0
Derivative assets	0	0
Financial liabilities
Derivative liabilities	0	0
Level 2
Financial assets
Investment securities available-for-sale, at fair value	1,054,877	787,429
Derivative assets	3,272	1,135
Financial liabilities
Derivative liabilities	3,272	1,135
Level 3
Financial assets
Investment securities available-for-sale, at fair value	0	0
Derivative assets	0	0
Financial liabilities
Derivative liabilities	0	0
Recurring basis
Financial assets
Investment securities available-for-sale, at fair value	1,054,877	787,429
Derivative assets	3,858	1,135
Financial liabilities
Derivative liabilities	3,858	1,135
Recurring basis | U.S. Treasury
Financial assets
Investment securities available-for-sale, at fair value	59,343
Recurring basis | Agency
Financial assets
Investment securities available-for-sale, at fair value	112,771	47,209
Recurring basis | Corporate
Financial assets
Investment securities available-for-sale, at fair value	101,594	79,546
Recurring basis | Municipal bonds
Financial assets
Investment securities available-for-sale, at fair value	231,201	232,128
Recurring basis | Collateralized mortgage obligation: residential
Financial assets
Investment securities available-for-sale, at fair value	25,206	33,781
Recurring basis | Mortgage-backed securities: residential
Financial assets
Investment securities available-for-sale, at fair value	524,762	394,765
Recurring basis | Level 1
Financial assets
Investment securities available-for-sale, at fair value	0	0
Derivative assets	0	0
Financial liabilities
Derivative liabilities	0	0
Recurring basis | Level 1 | U.S. Treasury
Financial assets
Investment securities available-for-sale, at fair value	0
Recurring basis | Level 1 | Agency
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 1 | Corporate
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 1 | Municipal bonds
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 1 | Collateralized mortgage obligation: residential
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 1 | Mortgage-backed securities: residential
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 2
Financial assets
Investment securities available-for-sale, at fair value	1,054,877	787,429
Derivative assets	3,858	1,135
Financial liabilities
Derivative liabilities	3,858	1,135
Recurring basis | Level 2 | U.S. Treasury
Financial assets
Investment securities available-for-sale, at fair value	59,343
Recurring basis | Level 2 | Agency
Financial assets
Investment securities available-for-sale, at fair value	112,771	47,209
Recurring basis | Level 2 | Corporate
Financial assets
Investment securities available-for-sale, at fair value	101,594	79,546
Recurring basis | Level 2 | Municipal bonds
Financial assets
Investment securities available-for-sale, at fair value	231,201	232,128
Recurring basis | Level 2 | Collateralized mortgage obligation: residential
Financial assets
Investment securities available-for-sale, at fair value	25,206	33,781
Recurring basis | Level 2 | Mortgage-backed securities: residential
Financial assets
Investment securities available-for-sale, at fair value	524,762	394,765
Recurring basis | Level 3
Financial assets
Investment securities available-for-sale, at fair value	0	0
Derivative assets	0	0
Financial liabilities
Derivative liabilities	0	0
Recurring basis | Level 3 | U.S. Treasury
Financial assets
Investment securities available-for-sale, at fair value	0
Recurring basis | Level 3 | Agency
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 3 | Corporate
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 3 | Municipal bonds
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 3 | Collateralized mortgage obligation: residential
Financial assets
Investment securities available-for-sale, at fair value	0	0
Recurring basis | Level 3 | Mortgage-backed securities: residential
Financial assets
Investment securities available-for-sale, at fair value	$ 0	$ 0