Fair Value Measurement and Fair Value of Financial Instruments (Carrying and Fair Value Estimates per the Fair Value Hierarchy of Financial Instruments Measured on a Nonrecurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financial assets:
Cash and cash equivalents	$ 3,080,042	$ 3,261,149
Interest-bearing deposits with banks	293,509	196,161
Resale agreements	860,000	860,000
Restricted equity securities, at cost	78,745	78,580
Loans held-for-investment, net	35,336,390	34,420,252
Financial liabilities:
FHLB advances	646,336	745,915
Repurchase agreements	450,000	200,000
Gross repurchase agreements	450,000	450,000
Carrying amount of repurchase agreements eligible for netting against resale agreements	0	250,000
Carrying Amount
Financial assets:
Cash and cash equivalents	3,080,042	3,261,149
Interest-bearing deposits with banks	293,509	196,161
Resale agreements	860,000	860,000
Restricted equity securities, at cost	78,745	78,580
Loans held-for-sale	1,594	434
Loans held-for-investment, net	35,336,390	34,420,252
Mortgage servicing rights	5,711	6,068
Accrued interest receivable	148,294	144,599
Financial liabilities:
Demand, checking, savings and money market deposits	28,720,425	27,109,951
Time deposits	9,966,533	10,214,308
Short-term borrowings	66,924	28,669
FHLB advances	646,336	745,915
Repurchase agreements	450,000	200,000
Long-term debt	147,169	147,101
Accrued interest payable	25,209	27,246
Estimated Fair Value
Financial assets:
Cash and cash equivalents	3,080,042	3,261,149
Interest-bearing deposits with banks	293,509	196,161
Resale agreements	867,872	856,025
Restricted equity securities, at cost	78,745	78,580
Loans held-for-sale	1,594	434
Loans held-for-investment, net	35,736,331	35,021,300
Mortgage servicing rights	7,926	8,199
Accrued interest receivable	148,294	144,599
Financial liabilities:
Demand, checking, savings and money market deposits	28,720,425	27,109,951
Time deposits	9,992,060	10,208,895
Short-term borrowings	66,924	28,669
FHLB advances	657,859	755,371
Repurchase agreements	470,230	232,597
Long-term debt	152,942	152,641
Accrued interest payable	25,209	27,246
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	3,080,042	3,261,149
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
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	293,509	196,161
Resale agreements	867,872	856,025
Restricted equity securities, at cost	78,745	78,580
Loans held-for-sale	1,594	434
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	148,294	144,599
Financial liabilities:
Demand, checking, savings and money market deposits	28,720,425	27,109,951
Time deposits	9,992,060	10,208,895
Short-term borrowings	66,924	28,669
FHLB advances	657,859	755,371
Repurchase agreements	470,230	232,597
Long-term debt	152,942	152,641
Accrued interest payable	25,209	27,246
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	35,736,331	35,021,300
Mortgage servicing rights	7,926	8,199
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
FHLB advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	$ 0	$ 0