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	Sep. 30, 2021	Dec. 31, 2020
Financial assets:
Cash and cash equivalents	$ 4,852,901	$ 4,017,971
Interest-bearing deposits with banks	855,162	809,728
Resale agreements	2,596,142	1,460,000
Restricted equity securities, at cost	77,200	83,046
Loans held-for-sale	0
Loans held-for-investment, net	39,921,301	37,770,972
Financial liabilities:
FHLB advances	248,898	652,612
Repurchase agreements	300,000	300,000
Carrying Amount
Financial assets:
Cash and cash equivalents	4,852,901	4,017,971
Interest-bearing deposits with banks	855,162	809,728
Resale agreements	2,596,142	1,460,000
Restricted equity securities, at cost	77,200	83,046
Loans held-for-sale	0	1,788
Loans held-for-investment, net	39,921,301	37,770,972
Mortgage servicing rights	5,620	5,522
Accrued interest receivable	151,826	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	45,117,548	35,862,403
Time deposits	8,238,642	9,000,349
Short-term borrowings		21,009
FHLB advances	248,898	652,612
Repurchase agreements	300,000	300,000
Long-term debt	147,586	147,376
Accrued interest payable	11,275	11,956
Estimated Fair Value
Financial assets:
Cash and cash equivalents	4,852,901	4,017,971
Interest-bearing deposits with banks	855,162	809,728
Resale agreements	2,580,845	1,464,635
Restricted equity securities, at cost	77,200	83,046
Loans held-for-sale	0	1,788
Loans held-for-investment, net	39,953,031	37,803,940
Mortgage servicing rights	8,858	8,435
Accrued interest receivable	151,826	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	45,117,548	35,862,403
Time deposits	8,248,717	9,016,884
Short-term borrowings		21,009
FHLB advances	250,635	659,631
Repurchase agreements	312,264	317,850
Long-term debt	149,625	150,131
Accrued interest payable	11,275	11,956
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	4,852,901	4,017,971
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
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	855,162	809,728
Resale agreements	2,580,845	1,464,635
Restricted equity securities, at cost	77,200	83,046
Loans held-for-sale	0	1,788
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	151,826	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	45,117,548	35,862,403
Time deposits	8,248,717	9,016,884
Short-term borrowings		21,009
FHLB advances	250,635	659,631
Repurchase agreements	312,264	317,850
Long-term debt	149,625	150,131
Accrued interest payable	11,275	11,956
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	39,953,031	37,803,940
Mortgage servicing rights	8,858	8,435
Accrued interest receivable	0	0
Financial liabilities:
Demand, checking, savings and money market deposits	0	0
Time deposits	0	0
Short-term borrowings		0
FHLB advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	$ 0	$ 0