Fair Value Measurements - Quantitative information about unobservable inputs for material assets and liabilities measured using fair value (Details) - USD ($)		3 Months Ended		12 Months Ended
		Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Debt securities held to maturity, estimated fair value		$ 1,967,996,000		$ 1,040,543,000
Fair Value, measurements, recurring | Other assets - SBIC investments
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Other assets, fair value				45,042,000
Fair Value, measurements, nonrecurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Debt securities held to maturity, estimated fair value		2,260,000				$ 2,013,000
Impaired loans, fair value	[1]	7,251,000				3,510,000
OREO, fair value		16,147,000				25,113,000
Fair Value, Inputs, Level 3
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Debt securities held to maturity, estimated fair value		1,033,648,000		1,040,543,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Interest rate contracts
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Interest rate contracts, fair value		2,608,000		2,416,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Other assets - MSRs
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Other assets, fair value		53,025,000		$ 49,597,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Other assets - SBIC investments
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Other assets, fair value		$ 47,987,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Minimum | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Closing rate		24.90%		24.90%
Cap grid rate		0.00%		0.20%
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Minimum | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Prepayment rate		0.00%		0.00%
Cost to service		$ 65		$ 65
Discount rate		8.00%		4.60%
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Maximum | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Closing rate		99.30%		99.30%
Cap grid rate		2.70%		2.30%
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Maximum | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Prepayment rate		27.50%		46.70%
Cost to service		$ 4,000		$ 4,000
Discount rate		9.70%		17.20%
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Average | Interest rate contracts | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Closing rate		66.90%		66.10%
Cap grid rate		1.00%		0.90%
Fair Value, Inputs, Level 3 | Fair Value, measurements, recurring | Average | Other assets - MSRs | Discounted cash flow
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Prepayment rate		7.70%		8.60%
Cost to service		$ 79		$ 81
Discount rate		8.10%		8.20%
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]
Transaction price		0.00%		0.00%
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Debt securities held to maturity, estimated fair value						2,013,000
Impaired loans, fair value	[1]	$ 7,251,000				3,510,000
OREO, fair value		16,147,000				$ 25,113,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Debt securities held to maturity
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Debt securities held to maturity, estimated fair value		2,260,000		$ 1,659,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Impaired loans
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Impaired loans, fair value		7,251,000		70,749,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | OREO
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
OREO, fair value		$ 16,147,000		$ 17,278,000
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Minimum | Impaired loans | Appraised value
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Appraised value rate		0.00%		0.00%
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Maximum | Impaired loans | Appraised value
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Appraised value rate		70.00%		100.00%
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]
Default rate		17.00%		4.80%
Loss severity rate		65.00%		70.60%
Prepayment rate		7.20%		5.10%
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Average | Impaired loans | Appraised value
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Appraised value rate		16.70%		19.20%
Fair Value, Inputs, Level 3 | Fair Value, measurements, nonrecurring | Average | OREO | Appraised value
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Appraised value rate		8.00%	[2]	8.00%	[3]

[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. Quantitative Information about Level 3 Fair Value Measurements Fair Value at Range of Unobservable Inputs December 31, 2017 Valuation Technique Unobservable Input(s) (Weighted Average) (In Thousands) Recurring fair value measurements: Interest rate contracts, net$2,416 Discounted cash flow Closing ratios (pull-through) 24.9% - 99.3% (66.1%) Cap grids 0.2% - 2.3% (0.9%)Other assets - MSRs49,597 Discounted cash flow Option adjusted spread 4.6% - 17.2% (8.2%) Constant prepayment rate or life speed 0.0% - 46.7% (8.6%) Cost to service $65 - $4,000 ($81)Other assets - SBIC investments45,042 Transaction price Transaction price N/ANonrecurring fair value measurements: Debt securities held to maturity$1,659 Discounted cash flow Prepayment rate 5.1% Default rate 4.8% Loss severity 70.6%Impaired loans70,749 Appraised value Appraised value 0.0% - 100.0% (19.2%)OREO17,278 Appraised value Appraised value 8.0% (1)

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