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 ($)	Sep. 30, 2020	Dec. 31, 2019
Financial assets:
Cash and cash equivalents	$ 4,506,941,000	$ 3,261,149,000
Interest-bearing deposits with banks	699,465,000	196,161,000
Resale agreements	1,210,000,000	860,000,000
Restricted equity securities, at cost	79,172,000	78,580,000
Loans held-for-investment, net	36,818,877,000	34,420,252,000
Financial liabilities:
FHLB advances	657,185,000	745,915,000
Repurchase agreements	348,063,000	200,000,000
Gross repurchase agreements	348,063,000	450,000,000
Carrying amount of repurchase agreements eligible for netting against resale agreements	0	250,000,000
Carrying Amount
Financial assets:
Cash and cash equivalents	4,506,941,000	3,261,149,000
Interest-bearing deposits with banks	699,465,000	196,161,000
Resale agreements	1,210,000,000	860,000,000
Restricted equity securities, at cost	79,172,000	78,580,000
Loans held-for-sale	4,148,000	434,000
Loans held-for-investment, net	36,818,877,000	34,420,252,000
Mortgage servicing rights	5,169,000	6,068,000
Accrued interest receivable	143,354,000	144,599,000
Financial liabilities:
Demand, checking, savings and money market deposits	32,611,382,000	27,109,951,000
Time deposits	9,069,173,000	10,214,308,000
Short-term borrowings	59,613,000	28,669,000
FHLB advances	657,185,000	745,915,000
Repurchase agreements	348,063,000	200,000,000
Long-term debt	1,574,765,000	147,101,000
Accrued interest payable	21,671,000	27,246,000
Estimated Fair Value
Financial assets:
Cash and cash equivalents	4,506,941,000	3,261,149,000
Interest-bearing deposits with banks	699,465,000	196,161,000
Resale agreements	1,215,801,000	856,025,000
Restricted equity securities, at cost	79,172,000	78,580,000
Loans held-for-sale	4,148,000	434,000
Loans held-for-investment, net	37,087,711,000	35,021,300,000
Mortgage servicing rights	7,511,000	8,199,000
Accrued interest receivable	143,354,000	144,599,000
Financial liabilities:
Demand, checking, savings and money market deposits	32,611,382,000	27,109,951,000
Time deposits	9,091,802,000	10,208,895,000
Short-term borrowings	59,613,000	28,669,000
FHLB advances	666,989,000	755,371,000
Repurchase agreements	368,098,000	232,597,000
Long-term debt	1,577,572,000	152,641,000
Accrued interest payable	21,671,000	27,246,000
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	4,506,941,000	3,261,149,000
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	699,465,000	196,161,000
Resale agreements	1,215,801,000	856,025,000
Restricted equity securities, at cost	79,172,000	78,580,000
Loans held-for-sale	4,148,000	434,000
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	143,354,000	144,599,000
Financial liabilities:
Demand, checking, savings and money market deposits	32,611,382,000	27,109,951,000
Time deposits	9,091,802,000	10,208,895,000
Short-term borrowings	59,613,000	28,669,000
FHLB advances	666,989,000	755,371,000
Repurchase agreements	368,098,000	232,597,000
Long-term debt	1,577,572,000	152,641,000
Accrued interest payable	21,671,000	27,246,000
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	37,087,711,000	35,021,300,000
Mortgage servicing rights	7,511,000	8,199,000
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