Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 9,937,236	$ 10,877,769
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	669,463	405,399
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3	13,577
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	10,606,696	11,269,591
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,117,867	1,941,283
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	45,768	18,934
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	12,084
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	1,163,635	1,948,133
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,051,686	2,176,275
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	79,604	32,608
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3	1,289
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	2,131,287	2,207,594
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	6,725,404	6,717,344
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	544,091	353,857
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	204
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	7,269,495	7,070,997
Other Investments [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	42,279	42,867
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	$ 42,279	$ 42,867