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, 2021	Dec. 31, 2020
Financial assets:
Cash and cash equivalents	$ 5,997,805	$ 4,017,971
Interest-bearing deposits with banks	830,279	809,728
Resale agreements	2,299,184	1,460,000
Restricted equity securities, at cost	76,931	83,046
Loans held-for-sale	1,800
Loans held-for-investment, net	39,485,775	37,770,972
Financial liabilities:
FHLB advances	248,464	652,612
Repurchase agreements	300,000	300,000
Carrying Amount
Financial assets:
Cash and cash equivalents	5,997,805	4,017,971
Interest-bearing deposits with banks	830,279	809,728
Resale agreements	2,299,184	1,460,000
Restricted equity securities, at cost	76,931	83,046
Loans held-for-sale	1,819	1,788
Loans held-for-investment, net	39,485,775	37,770,972
Mortgage servicing rights	5,373	5,522
Accrued interest receivable	154,810	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	44,151,817	35,862,403
Time deposits	8,430,758	9,000,349
Short-term borrowings		21,009
FHLB advances	248,464	652,612
Repurchase agreements	300,000	300,000
Long-term debt	147,515	147,376
Accrued interest payable	10,279	11,956
Estimated Fair Value
Financial assets:
Cash and cash equivalents	5,997,805	4,017,971
Interest-bearing deposits with banks	830,279	809,728
Resale agreements	2,290,097	1,464,635
Restricted equity securities, at cost	76,931	83,046
Loans held-for-sale	1,819	1,788
Loans held-for-investment, net	39,414,107	37,803,940
Mortgage servicing rights	8,788	8,435
Accrued interest receivable	154,810	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	44,151,817	35,862,403
Time deposits	8,442,834	9,016,884
Short-term borrowings		21,009
FHLB advances	250,871	659,631
Repurchase agreements	314,315	317,850
Long-term debt	150,232	150,131
Accrued interest payable	10,279	11,956
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	5,997,805	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	830,279	809,728
Resale agreements	2,290,097	1,464,635
Restricted equity securities, at cost	76,931	83,046
Loans held-for-sale	1,819	1,788
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	154,810	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	44,151,817	35,862,403
Time deposits	8,442,834	9,016,884
Short-term borrowings		21,009
FHLB advances	250,871	659,631
Repurchase agreements	314,315	317,850
Long-term debt	150,232	150,131
Accrued interest payable	10,279	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	39,414,107	37,803,940
Mortgage servicing rights	8,788	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