Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 10,259,473	$ 9,727,401
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	593,881	710,168
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	12,828	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	10,840,526	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,473,800	1,084,542
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	26,664	35,091
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,742	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	1,497,722	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,729,198	1,916,581
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	40,589	71,102
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,158	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	1,761,629	1,987,679
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	7,014,119	6,683,927
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	526,628	603,975
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,928	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	7,538,819	7,287,902
Other Investments [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	42,356	42,351
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	$ 42,356	$ 42,351