Fair Value Measurements - Quantitative information about unobservable inputs for material assets and liabilities measured using fair value (Details)		Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset		$ 879,744,000	$ 354,982,000
Debt securities held to maturity, estimated fair value		8,267,393,000	6,921,158,000
Fair Value, measurements, recurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset		41,340,000	18,643,000
Fair Value, Inputs, Level 3
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value		630,853,000	668,311,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset		14,324,000	3,088,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Interest rate contracts
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset		12,410,000	3,088,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Other assets - MSRs
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets		31,232,000	42,022,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Other assets - SBIC investments
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets		156,400,000	119,475,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Debt securities held to maturity
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value			2,177,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Impaired loans
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans			484,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | OREO
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned		$ 21,392,000	$ 21,583,000
Closing ratios | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Minimum | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset, measurement input		0.078	0.168
Closing ratios | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Maximum | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset, measurement input		0.998	1.000
Closing ratios | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Average | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset, measurement input		0.616	0.601
Cap grids | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Minimum | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset, measurement input		(0.002)	0.005
Cap grids | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Maximum | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset, measurement input		0.026	0.025
Cap grids | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Average | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset, measurement input		0.009	0.009
Discount rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Minimum | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		0.060	0.060
Discount rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Maximum | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		0.083	0.090
Discount rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Average | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		0.062	0.064
Constant prepayment rate or life speed | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Minimum | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		0.004	0.000
Constant prepayment rate or life speed | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Maximum | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		0.792	0.800
Constant prepayment rate or life speed | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Average | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		0.194	0.146
Cost to service | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Minimum | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		65	65
Cost to service | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Maximum | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		4,000	4,000
Cost to service | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Average | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		92	90
Transaction price | Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Average | Other assets - SBIC investments | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other assets, measurement input		0.000	0.000
Prepayment rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Minimum | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			13.700
Prepayment rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Maximum | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			14.700
Prepayment rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Average | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			0.142
Default rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Minimum | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			3.100
Default rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Maximum | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			4.900
Default rate | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Average | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			0.040
Loss severity | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Minimum | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			50.300
Loss severity | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Maximum | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			61.900
Loss severity | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Average | Debt securities held to maturity | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value, measurement input			0.561
Appraised value | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Minimum | Impaired loans | Appraised value
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans, measurement input			0.000
Appraised value | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Maximum | Impaired loans | Appraised value
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans, measurement input			0.700
Appraised value | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Average | Impaired loans | Appraised value
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans, measurement input			0.097
Appraised value | Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Average | OREO | Appraised value
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned, measurement input	[1]	0.080	0.080

[1] Represents discount to appraised value for estimated costs to sell.