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	Jun. 30, 2020	Dec. 31, 2019
Financial assets:
Cash and cash equivalents	$ 4,533,502	$ 3,261,149
Interest-bearing deposits with banks	531,591	196,161
Resale agreements	1,260,000	860,000
Restricted equity securities, at cost	78,963	78,580
Loans held-for-investment, net	36,597,341	34,420,252
Financial liabilities:
FHLB advances	656,759	745,915
Repurchase agreements	300,000	200,000
Gross repurchase agreements	300,000	450,000
Carrying amount of repurchase agreements eligible for netting against resale agreements	0	250,000
Carrying Amount
Financial assets:
Cash and cash equivalents	4,533,502	3,261,149
Interest-bearing deposits with banks	531,591	196,161
Resale agreements	1,260,000	860,000
Restricted equity securities, at cost	78,963	78,580
Loans held-for-sale	3,875	434
Loans held-for-investment, net	36,597,341	34,420,252
Mortgage servicing rights	5,363	6,068
Accrued interest receivable	149,595	144,599
Financial liabilities:
Demand, checking, savings and money market deposits	31,410,130	27,109,951
Time deposits	9,262,548	10,214,308
Short-term borrowings	252,851	28,669
FHLB advances	656,759	745,915
Repurchase agreements	300,000	200,000
Long-term debt	1,575,653	147,101
Accrued interest payable	23,007	27,246
Estimated Fair Value
Financial assets:
Cash and cash equivalents	4,533,502	3,261,149
Interest-bearing deposits with banks	531,591	196,161
Resale agreements	1,263,068	856,025
Restricted equity securities, at cost	78,963	78,580
Loans held-for-sale	3,875	434
Loans held-for-investment, net	36,714,052	35,021,300
Mortgage servicing rights	7,798	8,199
Accrued interest receivable	149,595	144,599
Financial liabilities:
Demand, checking, savings and money market deposits	31,410,130	27,109,951
Time deposits	9,295,199	10,208,895
Short-term borrowings	252,851	28,669
FHLB advances	667,996	755,371
Repurchase agreements	320,361	232,597
Long-term debt	1,578,801	152,641
Accrued interest payable	23,007	27,246
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	4,533,502	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	531,591	196,161
Resale agreements	1,263,068	856,025
Restricted equity securities, at cost	78,963	78,580
Loans held-for-sale	3,875	434
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	149,595	144,599
Financial liabilities:
Demand, checking, savings and money market deposits	31,410,130	27,109,951
Time deposits	9,295,199	10,208,895
Short-term borrowings	252,851	28,669
FHLB advances	667,996	755,371
Repurchase agreements	320,361	232,597
Long-term debt	1,578,801	152,641
Accrued interest payable	23,007	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	36,714,052	35,021,300
Mortgage servicing rights	7,798	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