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	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Cash and cash equivalents	$ 3,785,325	$ 3,001,377
Interest-bearing deposits with banks	134,000	371,000
Resale agreements	1,035,000	1,035,000
Restricted equity securities, at cost	74,736	74,069
Loans held-for-sale	0	275
Loans held-for-investment, net	32,545,392	32,073,867
Financial liabilities:
Short-term borrowings	39,550	57,638
FHLB advances	344,657	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	3,785,325	3,001,377
Interest-bearing deposits with banks	134,000	371,000
Resale agreements	1,035,000	1,035,000
Restricted equity securities, at cost	74,736	74,069
Loans held-for-sale		275
Loans held-for-investment, net	32,545,392	32,073,867
Mortgage servicing rights	7,754	7,836
Accrued interest receivable	157,335	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	26,427,303	26,370,562
Time deposits	9,846,669	9,069,066
Short-term borrowings	39,550	57,638
FHLB advances	344,657	326,172
Repurchase agreements	50,000	50,000
Long-term debt	146,900	146,835
Accrued interest payable	25,814	22,893
Estimated Fair Value
Financial assets:
Cash and cash equivalents	3,785,325	3,001,377
Interest-bearing deposits with banks	134,000	371,000
Resale agreements	1,025,288	1,016,724
Restricted equity securities, at cost	74,736	74,069
Loans held-for-sale		275
Loans held-for-investment, net	32,775,546	32,273,157
Mortgage servicing rights	11,099	11,427
Accrued interest receivable	157,335	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	26,427,303	26,370,562
Time deposits	9,876,954	9,084,597
Short-term borrowings	39,550	57,638
FHLB advances	352,610	334,793
Repurchase agreements	107,103	87,668
Long-term debt	152,531	152,556
Accrued interest payable	25,814	22,893
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	3,785,325	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
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	134,000	371,000
Resale agreements	1,025,288	1,016,724
Restricted equity securities, at cost	74,736	74,069
Loans held-for-sale		275
Loans held-for-investment, net	0	0
Mortgage servicing rights	0	0
Accrued interest receivable	157,335	146,262
Financial liabilities:
Demand, checking, savings and money market deposits	26,427,303	26,370,562
Time deposits	9,876,954	9,084,597
Short-term borrowings	39,550	57,638
FHLB advances	352,610	334,793
Repurchase agreements	107,103	87,668
Long-term debt	152,531	152,556
Accrued interest payable	25,814	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
Loans held-for-investment, net	32,775,546	32,273,157
Mortgage servicing rights	11,099	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