Fair Value Measurement and Fair Value of Financial Instruments (Carrying and Fair Values per the Fair Value Hierarchy of Certain Financial Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial assets:
Cash and cash equivalents	$ 3,001,377	$ 2,174,592	$ 1,878,503	$ 1,360,887
Interest-bearing deposits with banks	371,000	398,422
Resale agreements	1,035,000	1,050,000
Restricted equity securities, at cost	74,069	73,521
Loans held-for-sale	275	85
Loans held-for-investment, net	32,073,867	28,688,590
Financial liabilities:
Demand, checking, savings and money market deposits	26,370,562	25,974,314
Time deposits	9,069,066	5,640,749
Short-term borrowings	57,638	0
Federal Home Loan Bank (“FHLB”) advances	326,172	323,891
Repurchase agreements	50,000	50,000
Long-term debt	146,835	171,577
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	3,001,377	2,174,592
Interest-bearing deposits with banks	371,000	398,422
Resale agreements	1,035,000	1,050,000
Restricted equity securities, at cost	74,069	73,521
Loans held-for-sale	275	85
Loans held-for-investment, net	32,073,867	28,688,590
Branch assets held-for-sale		91,318
Mortgage servicing rights	7,836	7,771
Accrued interest receivable	146,262	121,719
Financial liabilities:
Demand, checking, savings and money market deposits	26,370,562	25,974,314
Time deposits	9,069,066	5,640,749
Short-term borrowings	57,638
Branch liability held-for-sale		605,111
Federal Home Loan Bank (“FHLB”) advances	326,172	323,891
Repurchase agreements	50,000	50,000
Long-term debt	146,835	171,577
Accrued interest payable	22,893	10,724
Fair Value Measurements
Financial assets:
Cash and cash equivalents	3,001,377	2,174,592
Interest-bearing deposits with banks	371,000	398,422
Resale agreements	1,016,724	1,035,158
Restricted equity securities, at cost	74,069	73,521
Loans held-for-sale	275	85
Loans held-for-investment, net	32,273,157	28,956,349
Branch assets held-for-sale		94,245
Mortgage servicing rights	11,427	11,324
Accrued interest receivable	146,262	121,719
Financial liabilities:
Demand, checking, savings and money market deposits	26,370,562	25,974,314
Time deposits	9,084,597	5,626,855
Short-term borrowings	57,638
Branch liability held-for-sale		643,937
Federal Home Loan Bank (“FHLB”) advances	334,793	335,901
Repurchase agreements	87,668	104,830
Long-term debt	152,556	171,673
Accrued interest payable	22,893	10,724
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	3,001,377	2,174,592
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
Branch assets held-for-sale		5,143
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
Branch liability held-for-sale		0
Federal Home Loan Bank (“FHLB”) advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	0	0
Fair Value Measurements | Significant Other Observable Inputs (Level 2)
Financial assets:
Cash and cash equivalents	0	0
Interest-bearing deposits with banks	371,000	398,422
Resale agreements	1,016,724	1,035,158
Restricted equity securities, at cost	74,069	73,521
Loans held-for-sale	275	85
Loans held-for-investment, net	0	0
Branch assets held-for-sale		10,970
Mortgage servicing rights	0	0
Accrued interest receivable	146,262	121,719
Financial liabilities:
Demand, checking, savings and money market deposits	26,370,562	25,974,314
Time deposits	9,084,597	5,626,855
Short-term borrowings	57,638
Branch liability held-for-sale		0
Federal Home Loan Bank (“FHLB”) advances	334,793	335,901
Repurchase agreements	87,668	104,830
Long-term debt	152,556	171,673
Accrued interest payable	22,893	10,724
Fair Value Measurements | 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	32,273,157	28,956,349
Branch assets held-for-sale		78,132
Mortgage servicing rights	11,427	11,324
Accrued interest receivable	0	0
Financial liabilities:
Demand, checking, savings and money market deposits	0	0
Time deposits	0	0
Short-term borrowings	0
Branch liability held-for-sale		643,937
Federal Home Loan Bank (“FHLB”) advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	$ 0	$ 0