Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Securities available‑for‑sale	$ 3,774,431	$ 3,223,830
Warrants Not Settleable in Cash, Fair Value Disclosure		5,497
Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	3,774,431	3,223,830
Derivative Asset, Fair Value of Collateral	1,873	694
Warrants Not Settleable in Cash, Fair Value Disclosure	5,161	5,497
Assets, Fair Value Disclosure, Recurring	3,779,592	3,230,021
Derivative Liability, Fair Value of Collateral	1,379	3,285
Level 1
Securities available‑for‑sale		2,080
Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	5,922	2,080
Derivative Asset, Fair Value of Collateral	0	0
Warrants Not Settleable in Cash, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	5,922	2,080
Derivative Liability, Fair Value of Collateral	0	0
Level 2
Securities available‑for‑sale		3,156,475
Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	3,703,526	3,156,475
Derivative Asset, Fair Value of Collateral	1,873	694
Warrants Not Settleable in Cash, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	3,703,526	3,157,169
Derivative Liability, Fair Value of Collateral	1,379	3,285
Level 3
Securities available‑for‑sale		65,275
Warrants Not Settleable in Cash, Fair Value Disclosure		5,497	$ 4,914
Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	64,983	65,275
Derivative Asset, Fair Value of Collateral	0	0
Warrants Not Settleable in Cash, Fair Value Disclosure	5,161	5,497
Assets, Fair Value Disclosure, Recurring	70,144	70,772
Derivative Liability, Fair Value of Collateral	0	0
Government agency and government-sponsored enterprise pass through securities [Member]
Securities available‑for‑sale	246,274	502,443
Government agency and government-sponsored enterprise pass through securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Government agency and government-sponsored enterprise pass through securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	246,274	502,443
Government agency and government-sponsored enterprise pass through securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member]
Securities available‑for‑sale	275,709	146,289
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	275,709	146,289
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Private Label Collateralized Mortgage Obligations [Member]
Securities available‑for‑sale	125,987	125,469
Collateralized Mortgage Obligations [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Collateralized Mortgage Obligations [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	103,113	68,567
Collateralized Mortgage Obligations [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	22,874	56,902
Municipal Securities [Member]
Securities available‑for‑sale	1,680,068	1,456,459
Municipal Securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Municipal Securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	1,680,068	1,456,459
Municipal Securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Commercial Mortgage Backed Securities [Member]
Securities available‑for‑sale	1,163,969	547,692
Commercial Mortgage Backed Securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Commercial Mortgage Backed Securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	1,163,969	547,692
Commercial Mortgage Backed Securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Corporate Debt Securities [Member]
Securities available‑for‑sale	19,295	47,509
Corporate Debt Securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Corporate Debt Securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	19,295	47,509
Corporate Debt Securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Collateralized Loan Obligations [Member]
Securities available‑for‑sale	7,015	156,887
Collateralized Loan Obligations [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Collateralized Loan Obligations [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	7,015	156,887
Collateralized Loan Obligations [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
SBA asset-backed securities [Member]
Securities available‑for‑sale	160,334	178,845
SBA asset-backed securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
SBA asset-backed securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	160,334	178,845
SBA asset-backed securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Other securities [Member]
Securities available‑for‑sale	95,780	62,237
Other securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	5,922	2,080
Other securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	47,749	51,784
Other securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	$ 42,109
Other securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member] | Square 1 Financial, Inc. [Member]
Securities available‑for‑sale		$ 8,373