Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Securities available for sale, at fair value	$ 1,177,606	$ 671,488
Derivative instruments	772	0
Mortgage loans held for sale in loans held for sale	252,025	108,316
Trust preferred securities
Financial assets:
Securities available for sale, at fair value	10,304	9,388
Other debt securities
Financial assets:
Securities available for sale, at fair value	34,965	17,290
Government agency mortgage backed securities | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	560,587	193,950
Government agency mortgage backed securities | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	21,735	31,170
Government agency collateralized mortgage obligations | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	307,228	176,639
Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	28,962	5,006
Recurring
Financial assets:
Securities available for sale, at fair value	1,177,606	671,488
Derivative instruments	11,116	5,977
Mortgage loans held for sale in loans held for sale	252,025	108,316
Total financial assets	1,440,747	785,781
Financial liabilities:
Derivative instruments	5,313	6,039
Total financial liabilities	5,313	6,039
Recurring | Interest rate swaps
Financial assets:
Derivative instruments	772
Financial liabilities:
Derivative instruments	781	2,536
Recurring | Interest rate contracts
Financial assets:
Derivative instruments	4,370	3,171
Financial liabilities:
Derivative instruments	4,370	3,171
Recurring | Interest rate lock commitments
Financial assets:
Derivative instruments	4,309	2,756
Financial liabilities:
Derivative instruments	83	4
Recurring | Forward commitments
Financial assets:
Derivative instruments	1,665	50
Financial liabilities:
Derivative instruments	79	328
Recurring | Trust preferred securities
Financial assets:
Securities available for sale, at fair value	10,304	9,388
Recurring | Other debt securities
Financial assets:
Securities available for sale, at fair value	34,965	17,290
Recurring | Obligations of other U.S. Government agencies and corporations
Financial assets:
Securities available for sale, at fair value	3,506	3,564
Recurring | Obligations of states and political subdivisions
Financial assets:
Securities available for sale, at fair value	210,319	234,481
Recurring | Government agency mortgage backed securities | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	560,587	193,950
Recurring | Government agency mortgage backed securities | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	21,735	31,170
Recurring | Government agency collateralized mortgage obligations | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	307,228	176,639
Recurring | Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	28,962	5,006
Recurring | Level 1
Financial assets:
Securities available for sale, at fair value	0	0
Derivative instruments	0	0
Mortgage loans held for sale in loans held for sale	0	0
Total financial assets	0	0
Financial liabilities:
Derivative instruments	0	0
Total financial liabilities	0	0
Recurring | Level 1 | Interest rate swaps
Financial assets:
Derivative instruments	0
Financial liabilities:
Derivative instruments	0	0
Recurring | Level 1 | Interest rate contracts
Financial assets:
Derivative instruments	0	0
Financial liabilities:
Derivative instruments	0	0
Recurring | Level 1 | Interest rate lock commitments
Financial assets:
Derivative instruments	0	0
Financial liabilities:
Derivative instruments	0	0
Recurring | Level 1 | Forward commitments
Financial assets:
Derivative instruments	0	0
Financial liabilities:
Derivative instruments	0	0
Recurring | Level 1 | Trust preferred securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 1 | Other debt securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 1 | Obligations of other U.S. Government agencies and corporations
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 1 | Obligations of states and political subdivisions
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 1 | Government agency mortgage backed securities | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 1 | Government agency mortgage backed securities | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 1 | Government agency collateralized mortgage obligations | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 1 | Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 2
Financial assets:
Securities available for sale, at fair value	1,167,302	662,100
Derivative instruments	11,116	5,977
Mortgage loans held for sale in loans held for sale	252,025	108,316
Total financial assets	1,430,443	776,393
Financial liabilities:
Derivative instruments	5,313	6,039
Total financial liabilities	5,313	6,039
Recurring | Level 2 | Interest rate swaps
Financial assets:
Derivative instruments	772
Financial liabilities:
Derivative instruments	781	2,536
Recurring | Level 2 | Interest rate contracts
Financial assets:
Derivative instruments	4,370	3,171
Financial liabilities:
Derivative instruments	4,370	3,171
Recurring | Level 2 | Interest rate lock commitments
Financial assets:
Derivative instruments	4,309	2,756
Financial liabilities:
Derivative instruments	83	4
Recurring | Level 2 | Forward commitments
Financial assets:
Derivative instruments	1,665	50
Financial liabilities:
Derivative instruments	79	328
Recurring | Level 2 | Trust preferred securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 2 | Other debt securities
Financial assets:
Securities available for sale, at fair value	34,965	17,290
Recurring | Level 2 | Obligations of other U.S. Government agencies and corporations
Financial assets:
Securities available for sale, at fair value	3,506	3,564
Recurring | Level 2 | Obligations of states and political subdivisions
Financial assets:
Securities available for sale, at fair value	210,319	234,481
Recurring | Level 2 | Government agency mortgage backed securities | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	560,587	193,950
Recurring | Level 2 | Government agency mortgage backed securities | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	21,735	31,170
Recurring | Level 2 | Government agency collateralized mortgage obligations | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	307,228	176,639
Recurring | Level 2 | Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	28,962	5,006
Recurring | Level 3
Financial assets:
Securities available for sale, at fair value	10,304	9,388
Derivative instruments	0	0
Mortgage loans held for sale in loans held for sale	0	0
Total financial assets	10,304	9,388
Financial liabilities:
Derivative instruments	0	0
Total financial liabilities	0	0
Recurring | Level 3 | Interest rate swaps
Financial assets:
Derivative instruments	0
Financial liabilities:
Derivative instruments	0	0
Recurring | Level 3 | Interest rate contracts
Financial assets:
Derivative instruments	0	0
Financial liabilities:
Derivative instruments	0	0
Recurring | Level 3 | Interest rate lock commitments
Financial assets:
Derivative instruments	0	0
Financial liabilities:
Derivative instruments	0	0
Recurring | Level 3 | Forward commitments
Financial assets:
Derivative instruments	0	0
Financial liabilities:
Derivative instruments	0	0
Recurring | Level 3 | Trust preferred securities
Financial assets:
Securities available for sale, at fair value	10,304	9,388
Recurring | Level 3 | Other debt securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 3 | Obligations of other U.S. Government agencies and corporations
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 3 | Obligations of states and political subdivisions
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 3 | Government agency mortgage backed securities | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 3 | Government agency mortgage backed securities | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 3 | Government agency collateralized mortgage obligations | Residential mortgage backed securities
Financial assets:
Securities available for sale, at fair value	0	0
Recurring | Level 3 | Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Financial assets:
Securities available for sale, at fair value	$ 0	$ 0