Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 9,827,059	$ 9,727,401
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	581,983	710,168
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	31,598	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	10,377,444	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,281,883	1,084,542
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	28,350	35,091
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,202	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	1,302,031	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,860,427	1,916,581
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	51,614	71,102
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	17,236	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	1,894,805	1,987,679
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	6,642,395	6,683,927
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	502,019	603,975
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,160	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, estimated fair value	7,138,254	7,287,902
Other Investments [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	42,354	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	$ 42,354	$ 42,351