Fair Value of Financial Instruments - Assets measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures
Total securities available for sale:	$ 805,511	$ 389,424
Recurring basis | Level 1
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Level 2
Fair Value Disclosures
Total securities available for sale:	787,429	380,963
Recurring basis | Level 3
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Agency | Level 1
Fair Value Disclosures
Total securities available for sale:	0
Recurring basis | Agency | Level 2
Fair Value Disclosures
Total securities available for sale:	47,209
Recurring basis | Agency | Level 3
Fair Value Disclosures
Total securities available for sale:	0
Recurring basis | Corporate | Level 1
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Corporate | Level 2
Fair Value Disclosures
Total securities available for sale:	79,546	37,642
Recurring basis | Corporate | Level 3
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Municipal bonds | Level 1
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Municipal bonds | Level 2
Fair Value Disclosures
Total securities available for sale:	232,128	118,803
Recurring basis | Municipal bonds | Level 3
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Collateralized mortgage obligation: residential | Level 1
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Collateralized mortgage obligation: residential | Level 2
Fair Value Disclosures
Total securities available for sale:	33,781	31,388
Recurring basis | Collateralized mortgage obligation: residential | Level 3
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Mortgage-backed securities: residential | Level 1
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Mortgage-backed securities: residential | Level 2
Fair Value Disclosures
Total securities available for sale:	394,765	193,130
Recurring basis | Mortgage-backed securities: residential | Level 3
Fair Value Disclosures
Total securities available for sale:	0	0
Recurring basis | Estimated Fair Value
Fair Value Disclosures
Total securities available for sale:	787,429	380,963
Recurring basis | Estimated Fair Value | Agency
Fair Value Disclosures
Total securities available for sale:	47,209
Recurring basis | Estimated Fair Value | Corporate
Fair Value Disclosures
Total securities available for sale:	79,546	37,642
Recurring basis | Estimated Fair Value | Municipal bonds
Fair Value Disclosures
Total securities available for sale:	232,128	118,803
Recurring basis | Estimated Fair Value | Collateralized mortgage obligation: residential
Fair Value Disclosures
Total securities available for sale:	33,781	31,388
Recurring basis | Estimated Fair Value | Mortgage-backed securities: residential
Fair Value Disclosures
Total securities available for sale:	$ 394,765	$ 193,130