Fair Value Measurement and Fair Value of Financial Instruments (Carrying and Fair Values Estimates per the Fair Value Hierarchy of Financial Instruments on a nonrecurring basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial assets:
Cash and cash equivalents	$ 3,261,149	$ 3,001,377
Interest-bearing deposits with banks	196,161	371,000
Resale agreements	860,000	1,035,000
Restricted equity securities, at cost	78,580	74,069
Loans held-for-sale	434	275
Loans held-for-investment, net	34,420,252	32,073,867
Financial liabilities:
Time deposits	10,214,308
Short-term borrowings	28,669	57,638
Federal Home Loan Bank (“FHLB”) advances	745,915	326,172
Repurchase agreements	200,000	50,000
Long-term debt		146,835
Gross repurchase agreements	450,000	450,000
Carrying amount of repurchase agreements eligible for netting against resale agreements	250,000	400,000
Carrying Amount
Financial assets:
Cash and cash equivalents	3,261,149	3,001,377
Interest-bearing deposits with banks	196,161	371,000
Resale agreements	860,000	1,035,000
Restricted equity securities, at cost	78,580	74,069
Loans held-for-sale	434	275
Loans held-for-investment, net	34,420,252	32,073,867
Mortgage servicing rights	6,068	7,836
Accrued interest receivable	144,599	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	27,109,951	26,370,562
Time deposits	10,214,308	9,069,066
Short-term borrowings	28,669	57,638
Federal Home Loan Bank (“FHLB”) advances	745,915	326,172
Repurchase agreements	200,000	50,000
Long-term debt	147,101	146,835
Accrued interest payable	27,246	22,893
Estimated Fair Value
Financial assets:
Cash and cash equivalents	3,261,149	3,001,377
Interest-bearing deposits with banks	196,161	371,000
Resale agreements	856,025	1,016,724
Restricted equity securities, at cost	78,580	74,069
Loans held-for-sale	434	275
Loans held-for-investment, net	35,021,300	32,273,157
Mortgage servicing rights	8,199	11,427
Accrued interest receivable	144,599	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	27,109,951	26,370,562
Time deposits	10,208,895	9,084,597
Short-term borrowings	28,669	57,638
Federal Home Loan Bank (“FHLB”) advances	755,371	334,793
Repurchase agreements	232,597	87,668
Long-term debt	152,641	152,556
Accrued interest payable	27,246	22,893
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	3,261,149	3,001,377
Interest-bearing deposits with banks	0	0
Resale agreements	0	0
Restricted equity securities, at cost	0	0
Loans held-for-sale	0	0
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	0	0
Financial liabilities:
Demand, checking, savings and money market deposits	0	0
Time deposits	0	0
Short-term borrowings	0	0
Federal Home Loan Bank (“FHLB”) advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	0	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Cash and cash equivalents	0	0
Interest-bearing deposits with banks	196,161	371,000
Resale agreements	856,025	1,016,724
Restricted equity securities, at cost	78,580	74,069
Loans held-for-sale	434	275
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	144,599	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	27,109,951	26,370,562
Time deposits	10,208,895	9,084,597
Short-term borrowings	28,669	57,638
Federal Home Loan Bank (“FHLB”) advances	755,371	334,793
Repurchase agreements	232,597	87,668
Long-term debt	152,641	152,556
Accrued interest payable	27,246	22,893
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Cash and cash equivalents	0	0
Interest-bearing deposits with banks	0	0
Resale agreements	0	0
Restricted equity securities, at cost	0	0
Loans held-for-sale	0	0
Loans held-for-investment, net	35,021,300	32,273,157
Mortgage servicing rights	8,199	11,427
Accrued interest receivable	0	0
Financial liabilities:
Demand, checking, savings and money market deposits	0	0
Time deposits	0	0
Short-term borrowings	0	0
Federal Home Loan Bank (“FHLB”) advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	$ 0	$ 0