Fair Value of Assets & Liabilities - Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	$ 1,930,991	$ 1,416,345
Loans held-for-sale (elected fair value)	16,418	20,881
Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans held-for-sale (elected fair value)	16,418	20,881
Securities available-for-sale	4,608,383	4,931,402
Total other assets	117,529	121,456
Total assets	6,673,321	6,490,084
Total other liabilities	170,324	85,061
Total liabilities	910,018	723,576
Recurring | Deferred compensation mutual funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	40,912	39,822
Recurring | Derivatives, forwards and futures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	20,013	10,161
Total other liabilities	18,770	9,535
Recurring | Derivatives, interest rate contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	34,333	71,473
Total other liabilities	117,236	69,842
Recurring | Derivatives, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	130
Total other liabilities	34,318	5,684
Recurring | U.S. treasuries
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	98	99
Recurring | Government agency issued MBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	2,414,877	2,577,376
Recurring | Government agency issued CMO
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	1,991,184	2,269,858
Recurring | Other U.S. government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	113,088
Recurring | States and municipalities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	23,701
Recurring | Corporates and other debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	55,074	55,782
Recurring | Equity, mutual funds, and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale		27,017
Total other assets	22,141
Recurring | Interest-only strips (elected fair value)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	10,361	1,270
Recurring | Trading securities—fixed income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	1,929,399	1,414,194
Total trading liabilities—fixed income	739,694	638,515
Recurring | Trading securities—fixed income | U.S. treasuries
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	121,490	128,995
Total trading liabilities—fixed income	574,897	506,679
Recurring | Trading securities—fixed income | Government agency issued MBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	235,403	227,038
Recurring | Trading securities—fixed income | Government agency issued CMO
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	568,344	275,014
Recurring | Trading securities—fixed income | Other U.S. government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	43,642	54,699
Recurring | Trading securities—fixed income | States and municipalities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	54,669	34,573
Total trading liabilities—fixed income	110
Recurring | Trading securities—fixed income | Corporates and other debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	866,128	693,877
Total trading liabilities—fixed income	164,687	131,836
Recurring | Trading securities—fixed income | Equity, mutual funds, and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	39,723	(2)
Recurring | Trading securities—mortgage banking
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	1,592	2,151
Recurring | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans held-for-sale (elected fair value)	0	0
Securities available-for-sale	0	27,017
Total other assets	83,066	49,983
Total assets	83,066	77,000
Total other liabilities	18,770	9,535
Total liabilities	18,770	9,535
Recurring | Level 1 | Deferred compensation mutual funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	40,912	39,822
Recurring | Level 1 | Derivatives, forwards and futures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	20,013	10,161
Total other liabilities	18,770	9,535
Recurring | Level 1 | Derivatives, interest rate contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	0	0
Total other liabilities	0	0
Recurring | Level 1 | Derivatives, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	0
Total other liabilities	0	0
Recurring | Level 1 | U.S. treasuries
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 1 | Government agency issued MBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 1 | Government agency issued CMO
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 1 | Other U.S. government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0
Recurring | Level 1 | States and municipalities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0
Recurring | Level 1 | Corporates and other debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 1 | Equity, mutual funds, and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale		27,017
Total other assets	22,141
Recurring | Level 1 | Interest-only strips (elected fair value)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 1 | Trading securities—fixed income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Total trading liabilities—fixed income	0	0
Recurring | Level 1 | Trading securities—fixed income | U.S. treasuries
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Total trading liabilities—fixed income	0	0
Recurring | Level 1 | Trading securities—fixed income | Government agency issued MBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 1 | Trading securities—fixed income | Government agency issued CMO
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 1 | Trading securities—fixed income | Other U.S. government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 1 | Trading securities—fixed income | States and municipalities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Total trading liabilities—fixed income	0
Recurring | Level 1 | Trading securities—fixed income | Corporates and other debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Total trading liabilities—fixed income	0	0
Recurring | Level 1 | Trading securities—fixed income | Equity, mutual funds, and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 1 | Trading securities—mortgage banking
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans held-for-sale (elected fair value)	182	1,955
Securities available-for-sale	4,598,022	4,903,115
Total other assets	34,463	71,473
Total assets	6,562,066	6,390,737
Total other liabilities	117,342	69,881
Total liabilities	857,036	708,396
Recurring | Level 2 | Deferred compensation mutual funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	0	0
Recurring | Level 2 | Derivatives, forwards and futures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets		0
Total other liabilities	0	0
Recurring | Level 2 | Derivatives, interest rate contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	34,333	71,473
Total other liabilities	117,236	69,842
Recurring | Level 2 | Derivatives, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	130
Total other liabilities	106	39
Recurring | Level 2 | U.S. treasuries
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	98	99
Recurring | Level 2 | Government agency issued MBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	2,414,877	2,577,376
Recurring | Level 2 | Government agency issued CMO
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	1,991,184	2,269,858
Recurring | Level 2 | Other U.S. government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	113,088
Recurring | Level 2 | States and municipalities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	23,701
Recurring | Level 2 | Corporates and other debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	55,074	55,782
Recurring | Level 2 | Equity, mutual funds, and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale		0
Total other assets	0
Recurring | Level 2 | Interest-only strips (elected fair value)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 2 | Trading securities—fixed income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	1,929,399	1,414,194
Total trading liabilities—fixed income	739,694	638,515
Recurring | Level 2 | Trading securities—fixed income | U.S. treasuries
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	121,490	128,995
Total trading liabilities—fixed income	574,897	506,679
Recurring | Level 2 | Trading securities—fixed income | Government agency issued MBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	235,403	227,038
Recurring | Level 2 | Trading securities—fixed income | Government agency issued CMO
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	568,344	275,014
Recurring | Level 2 | Trading securities—fixed income | Other U.S. government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	43,642	54,699
Recurring | Level 2 | Trading securities—fixed income | States and municipalities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	54,669	34,573
Total trading liabilities—fixed income	110
Recurring | Level 2 | Trading securities—fixed income | Corporates and other debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	866,128	693,877
Total trading liabilities—fixed income	164,687	131,836
Recurring | Level 2 | Trading securities—fixed income | Equity, mutual funds, and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	39,723	(2)
Recurring | Level 2 | Trading securities—mortgage banking
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans held-for-sale (elected fair value)	16,236	18,926
Securities available-for-sale	10,361	1,270
Total other assets	0	0
Total assets	28,189	22,347
Total other liabilities	34,212	5,645
Total liabilities	34,212	5,645
Recurring | Level 3 | Deferred compensation mutual funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	0	0
Recurring | Level 3 | Derivatives, forwards and futures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	0	0
Total other liabilities	0	0
Recurring | Level 3 | Derivatives, interest rate contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	0	0
Total other liabilities	0	0
Recurring | Level 3 | Derivatives, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total other assets	0
Total other liabilities	34,212	5,645
Recurring | Level 3 | U.S. treasuries
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 3 | Government agency issued MBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 3 | Government agency issued CMO
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 3 | Other U.S. government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0
Recurring | Level 3 | States and municipalities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0
Recurring | Level 3 | Corporates and other debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	0	0
Recurring | Level 3 | Equity, mutual funds, and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale		0
Total other assets	0
Recurring | Level 3 | Interest-only strips (elected fair value)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Securities available-for-sale	10,361	1,270
Recurring | Level 3 | Trading securities—fixed income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Total trading liabilities—fixed income	0	0
Recurring | Level 3 | Trading securities—fixed income | U.S. treasuries
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Total trading liabilities—fixed income	0	0
Recurring | Level 3 | Trading securities—fixed income | Government agency issued MBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 3 | Trading securities—fixed income | Government agency issued CMO
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 3 | Trading securities—fixed income | Other U.S. government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 3 | Trading securities—fixed income | States and municipalities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Total trading liabilities—fixed income	0
Recurring | Level 3 | Trading securities—fixed income | Corporates and other debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Total trading liabilities—fixed income	0	0
Recurring | Level 3 | Trading securities—fixed income | Equity, mutual funds, and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	0	0
Recurring | Level 3 | Trading securities—mortgage banking
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Trading securities	$ 1,592	$ 2,151