Fair Value Measurements - Quantitative information about unobservable inputs for material assets and liabilities measured using fair value (Details)		Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset		$ 877,940,000	$ 354,982,000
Debt securities held to maturity, estimated fair value		9,078,298,000	6,921,158,000
Fair Value, measurements, recurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative asset		37,598,000	18,643,000
Fair Value, measurements, nonrecurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value				$ 1,072,000
Impaired loans	[1]			3,404,000
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		623,563,000	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		20,743,000	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		20,743,000	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		29,035,000	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		152,784,000	119,475,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt securities held to maturity, estimated fair value				1,072,000
Impaired loans	[1]			$ 3,404,000
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		$ 14,871,000	$ 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		0.062	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		0.999	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		0.642	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		(0.001)	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		0.021	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
Option Adjusted Spread | 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
Option Adjusted Spread | 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
Option Adjusted Spread | 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		0.000	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		0.823	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		0.209	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, cost to service weighted average, 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, cost to service weighted average, 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, cost to service weighted average, measurement input		$ 105	$ 90
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			0.137
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			0.147
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			0.031
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			0.049
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			0.503
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			0.619
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	[2]	0.080	0.080

[1] Total gains (losses) represent charge-offs on impaired loans for which adjustments are based on the appraised value of the collateral.

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