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, 2021	Dec. 31, 2020
Financial assets:
Cash and cash equivalents	$ 4,619,133	$ 4,017,971
Interest-bearing deposits with banks	741,923	809,728
Resale agreements	2,160,038	1,460,000
Restricted equity securities, at cost	83,250	83,046
Loans held-for-investment, net	38,981,242	37,770,972
Financial liabilities:
FHLB advances	653,035	652,612
Repurchase agreements	300,000	300,000
Carrying Amount
Financial assets:
Cash and cash equivalents	4,619,133	4,017,971
Interest-bearing deposits with banks	741,923	809,728
Resale agreements	2,160,038	1,460,000
Restricted equity securities, at cost	83,250	83,046
Loans held-for-sale		1,788
Loans held-for-investment, net	38,981,242	37,770,972
Mortgage servicing rights	5,506	5,522
Accrued interest receivable	149,953	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	40,748,303	35,862,403
Time deposits	8,798,833	9,000,349
Short-term borrowings		21,009
FHLB advances	653,035	652,612
Repurchase agreements	300,000	300,000
Long-term debt	147,445	147,376
Accrued interest payable	9,966	11,956
Estimated Fair Value
Financial assets:
Cash and cash equivalents	4,619,133	4,017,971
Interest-bearing deposits with banks	741,923	809,728
Resale agreements	2,152,392	1,464,635
Restricted equity securities, at cost	83,250	83,046
Loans held-for-sale		1,788
Loans held-for-investment, net	38,922,550	37,803,940
Mortgage servicing rights	8,740	8,435
Accrued interest receivable	149,953	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	40,748,303	35,862,403
Time deposits	8,811,048	9,016,884
Short-term borrowings		21,009
FHLB advances	657,460	659,631
Repurchase agreements	316,099	317,850
Long-term debt	151,015	150,131
Accrued interest payable	9,966	11,956
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	4,619,133	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
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	741,923	809,728
Resale agreements	2,152,392	1,464,635
Restricted equity securities, at cost	83,250	83,046
Loans held-for-sale		1,788
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	149,953	150,140
Financial liabilities:
Demand, checking, savings and money market deposits	40,748,303	35,862,403
Time deposits	8,811,048	9,016,884
Short-term borrowings		21,009
FHLB advances	657,460	659,631
Repurchase agreements	316,099	317,850
Long-term debt	151,015	150,131
Accrued interest payable	9,966	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
Loans held-for-investment, net	38,922,550	37,803,940
Mortgage servicing rights	8,740	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