Fair Value Measurements (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	$ 3,820,333	$ 3,774,431
Equity warrants		5,161
Fair Value, Estimate Not Practicable, Equity Method Investments	6,552
Derivative Liability	468	1,379
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		5,922
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		3,703,526
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		64,983
Government agency and government-sponsored enterprise pass through securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	249,831	246,274
Government agency and government-sponsored enterprise collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	569,605	275,709
Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	1,271,583	1,680,068
Government agency and government-sponsored enterprise commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	1,077,005	1,163,969
US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	397,631	0
SBA asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	67,586	160,334
Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	58,452	88,710
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	18,190	19,295
Collateralized loan obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	7,015
Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	7,070
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	3,820,333	3,774,431
Equity warrants	4,962	5,161
Other derivative assets	2,072	1,873
Total recurring assets	3,833,919	3,781,465
Derivative liabilities	468	1,379
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	397,631	5,922
Equity warrants	0	0
Other derivative assets	0	0
Fair Value, Estimate Not Practicable, Equity Method Investments	6,552
Total recurring assets	404,183	5,922
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	3,380,480	3,703,526
Equity warrants	0	0
Other derivative assets	2,072	1,873
Fair Value, Estimate Not Practicable, Equity Method Investments	0
Total recurring assets	3,382,552	3,705,399
Derivative liabilities	468	1,379
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	42,222	64,983
Equity warrants	4,962	5,161
Other derivative assets	0	0
Fair Value, Estimate Not Practicable, Equity Method Investments	0
Total recurring assets	47,184	70,144
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise pass through securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	249,831	246,274
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise pass through securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise pass through securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	249,831	246,274
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise pass through securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	569,605	275,709
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise collateralized mortgage obligations | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise collateralized mortgage obligations | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	569,605	275,709
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise collateralized mortgage obligations | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Private Label Collateralized Mortgage Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	110,450	125,987
Fair Value, Measurements, Recurring | Private Label Collateralized Mortgage Obligations | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Private Label Collateralized Mortgage Obligations | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	97,327	103,113
Fair Value, Measurements, Recurring | Private Label Collateralized Mortgage Obligations | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	13,123	22,874
Fair Value, Measurements, Recurring | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	1,271,583	1,680,068
Fair Value, Measurements, Recurring | Municipal securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Municipal securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	1,271,583	1,680,068
Fair Value, Measurements, Recurring | Municipal securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	1,077,005	1,163,969
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise commercial mortgage-backed securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise commercial mortgage-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	1,077,005	1,163,969
Fair Value, Measurements, Recurring | Government agency and government-sponsored enterprise commercial mortgage-backed securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	397,631
Fair Value, Measurements, Recurring | US Treasury Securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	397,631
Fair Value, Measurements, Recurring | US Treasury Securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0
Fair Value, Measurements, Recurring | US Treasury Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0
Fair Value, Measurements, Recurring | SBA asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	67,586	160,334
Fair Value, Measurements, Recurring | SBA asset-backed securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | SBA asset-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	67,586	160,334
Fair Value, Measurements, Recurring | SBA asset-backed securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	58,452	88,710
Fair Value, Measurements, Recurring | Asset-backed | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Asset-backed | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	29,353	46,601
Fair Value, Measurements, Recurring | Asset-backed | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	29,099
Fair Value, Measurements, Recurring | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	18,190	19,295
Fair Value, Measurements, Recurring | Corporate debt securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	18,190	19,295
Fair Value, Measurements, Recurring | Corporate debt securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring | Collateralized loan obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		7,015
Fair Value, Measurements, Recurring | Collateralized loan obligations | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		0
Fair Value, Measurements, Recurring | Collateralized loan obligations | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		7,015
Fair Value, Measurements, Recurring | Collateralized loan obligations | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		0
Fair Value, Measurements, Recurring | Equity Securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		5,922
Fair Value, Measurements, Recurring | Equity Securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		1,148
Fair Value, Measurements, Recurring | Equity Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		0
Square 1 Financial, Inc. | Fair Value, Measurements, Recurring | Asset-backed | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available‑for‑sale		$ 42,109
Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 7,100