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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial assets:
Cash and cash equivalents	$ 2,174,592	$ 1,878,503	$ 1,360,887	$ 1,039,885
Interest-bearing deposits with banks	398,422	323,148
Resale agreements	1,050,000	2,000,000
Held-to-maturity investment security	0	143,971
Restricted equity securities, at cost	73,521	72,775
Loans held-for-sale	85	23,076
Loans held-for-investment, net	28,688,590	25,242,619
Branch assets held-for-sale	91,318	0
Customer deposits:
Demand, checking, savings and money market deposits	25,974,314	24,275,714
Time deposits	5,640,749	5,615,269
Short-term borrowings	0	60,050
Federal Home Loan Bank (“FHLB”) advances	323,891	321,643
Repurchase agreements	50,000	350,000
Long-term debt	171,577	186,327
Carrying amount of repurchase agreements eligible for netting against resale agreements	400,000	100,000
Gross repurchase agreements	450,000	450,000
Carrying Amount
Financial assets:
Cash and cash equivalents	2,174,592	1,878,503
Interest-bearing deposits with banks	398,422	323,148
Resale agreements	1,050,000	2,000,000
Held-to-maturity investment security		143,971
Restricted equity securities, at cost	73,521	72,775
Loans held-for-sale	85	23,076
Loans held-for-investment, net	28,688,590	25,242,619
Branch assets held-for-sale	91,318
Accrued interest receivable	121,719	100,524
Customer deposits:
Demand, checking, savings and money market deposits	25,974,314	24,275,714
Time deposits	5,640,749	5,615,269
Branch liability held-for-sale	605,111
Short-term borrowings		60,050
Federal Home Loan Bank (“FHLB”) advances	323,891	321,643
Repurchase agreements	50,000	350,000
Long-term debt	171,577	186,327
Accrued interest payable	10,724	9,440
Fair Value Measurements
Financial assets:
Cash and cash equivalents	2,174,592	1,878,503
Interest-bearing deposits with banks	398,422	323,148
Resale agreements	1,035,158	1,980,457
Held-to-maturity investment security		144,593
Restricted equity securities, at cost	73,521	72,775
Loans held-for-sale	85	23,076
Loans held-for-investment, net	28,956,349	24,915,143
Branch assets held-for-sale	94,245
Accrued interest receivable	121,719	100,524
Customer deposits:
Demand, checking, savings and money market deposits	25,974,314	24,275,714
Time deposits	5,626,855	5,611,746
Branch liability held-for-sale	643,937
Short-term borrowings		60,050
Federal Home Loan Bank (“FHLB”) advances	335,901	334,859
Repurchase agreements	104,830	411,368
Long-term debt	171,673	186,670
Accrued interest payable	10,724	9,440
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	2,174,592	1,878,503
Interest-bearing deposits with banks	0	0
Resale agreements	0	0
Held-to-maturity investment security		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
Accrued interest receivable	0	0
Customer deposits:
Demand, checking, savings and money market deposits	0	0
Time deposits	0	0
Branch liability held-for-sale	0
Short-term borrowings		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	398,422	323,148
Resale agreements	1,035,158	1,980,457
Held-to-maturity investment security		0
Restricted equity securities, at cost	73,521	72,775
Loans held-for-sale	85	23,076
Loans held-for-investment, net	0	0
Branch assets held-for-sale	10,970
Accrued interest receivable	121,719	100,524
Customer deposits:
Demand, checking, savings and money market deposits	25,974,314	24,275,714
Time deposits	5,626,855	5,611,746
Branch liability held-for-sale	0
Short-term borrowings		60,050
Federal Home Loan Bank (“FHLB”) advances	335,901	334,859
Repurchase agreements	104,830	411,368
Long-term debt	171,673	186,670
Accrued interest payable	10,724	9,440
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
Held-to-maturity investment security		144,593
Restricted equity securities, at cost	0	0
Loans held-for-sale	0	0
Loans held-for-investment, net	28,956,349	24,915,143
Branch assets held-for-sale	78,132
Accrued interest receivable	0	0
Customer deposits:
Demand, checking, savings and money market deposits	0	0
Time deposits	0	0
Branch liability held-for-sale	643,937
Short-term borrowings		0
Federal Home Loan Bank (“FHLB”) advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	$ 0	$ 0