Fair Value Measurement and Fair Value of Financial Instruments (Carrying and Fair Values per the Fair Value Hierarchy of Financial Instruments Measured on a Nonrecurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial assets:
Cash and cash equivalents	$ 3,621,614	$ 3,001,377
Interest-bearing deposits with banks	150,273	371,000
Resale agreements	1,010,000	1,035,000
Restricted equity securities, at cost	78,093	74,069
Loans held-for-sale	3,879	275
Loans held-for-investment, net	33,399,752	32,073,867
Financial liabilities:
Short-term borrowings	19,972	57,638
FHLB advances	745,074	326,172
Repurchase agreements	50,000	50,000
Long-term debt		146,835
Carrying amount of repurchase agreements eligible for netting against resale agreements	400,000	400,000
Gross repurchase agreements	450,000	450,000
Carrying Amount
Financial assets:
Cash and cash equivalents	3,621,614	3,001,377
Interest-bearing deposits with banks	150,273	371,000
Resale agreements	1,010,000	1,035,000
Restricted equity securities, at cost	78,093	74,069
Loans held-for-sale	3,879	275
Loans held-for-investment, net	33,399,752	32,073,867
Mortgage servicing rights	6,749	7,836
Accrued interest receivable	151,700	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	25,880,826	26,370,562
Time deposits	10,596,716	9,069,066
Short-term borrowings	19,972	57,638
FHLB advances	745,074	326,172
Repurchase agreements	50,000	50,000
Long-term debt	146,966	146,835
Accrued interest payable	27,684	22,893
Estimated Fair Value
Financial assets:
Cash and cash equivalents	3,621,614	3,001,377
Interest-bearing deposits with banks	150,273	371,000
Resale agreements	1,009,457	1,016,724
Restricted equity securities, at cost	78,093	74,069
Loans held-for-sale	3,879	275
Loans held-for-investment, net	33,605,951	32,273,157
Mortgage servicing rights	9,379	11,427
Accrued interest receivable	151,700	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	25,880,826	26,370,562
Time deposits	10,626,897	9,084,597
Short-term borrowings	19,972	57,638
FHLB advances	754,240	334,793
Repurchase agreements	109,525	87,668
Long-term debt	152,478	152,556
Accrued interest payable	27,684	22,893
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	3,621,614	3,001,377
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	150,273	371,000
Resale agreements	1,009,457	1,016,724
Restricted equity securities, at cost	78,093	74,069
Loans held-for-sale	3,879	275
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	151,700	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	25,880,826	26,370,562
Time deposits	10,626,897	9,084,597
Short-term borrowings	19,972	57,638
FHLB advances	754,240	334,793
Repurchase agreements	109,525	87,668
Long-term debt	152,478	152,556
Accrued interest payable	27,684	22,893
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	33,605,951	32,273,157
Mortgage servicing rights	9,379	11,427
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