Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 9,914,688	$ 10,877,769
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	689,835	405,399
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	43	13,577
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	10,604,480	11,269,591
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,084,210	1,941,283
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	40,755	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	1,124,965	1,948,133
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,092,184	2,176,275
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	72,021	32,608
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	43	1,289
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	2,164,162	2,207,594
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	6,695,946	6,717,344
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	577,059	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	7,273,005	7,070,997
Other Investments [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	42,348	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	$ 42,348	$ 42,867