Accounting Policies and Basis of Presentation Accounting Policies and Basis of Presentation (Recently Adopted Accounting Standards) (Details) - USD ($)		3 Months Ended		9 Months Ended
	Mar. 12, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock issued (in shares)				339,937,000
Preferred stock, par value (in dollars per share)		$ 0.01		$ 0.01				$ 0.01
Proceeds from Issuance of Preferred Stock and Preference Stock				$ 345,000,000	$ 0
Stock repurchased during period (in shares)		117,700	295,900	813,090	744,400
Stock repurchased during period		$ 47,060,000	$ 135,400,000	$ 333,755,000	$ 325,900,000
Stock repurchased during period (in USD per share)		$ 399.82	$ 457.50	$ 410.48	$ 437.84
Forecast period (years)				2 years
Expected credit losses on SBA-PPP loans		$ 0		$ 0
Reserve for unfunded commitments		14,000,000.0		14,000,000.0
Retained earnings		3,738,417,000		3,738,417,000				$ 3,658,197,000
Allowance for credit loss		223,936,000	$ 226,825,000	223,936,000	$ 226,825,000	$ 222,450,000		225,141,000	$ 226,583,000	$ 223,712,000
Allowance for credit loss on debt securities held to maturity		0		0
Loans and leases, not acquired in a transfer accounted for as debt securities		32,349,266,000		32,349,266,000				28,322,780,000
Loans and leases		336,382,000	315,012,000	988,029,000	909,167,000
Impairment loss		0
Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans and leases, not acquired in a transfer accounted for as debt securities		22,613,932,000		22,613,932,000				$ 18,552,002,000
SBA-PPP | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans and leases, not acquired in a transfer accounted for as debt securities		3,112,676,000		3,112,676,000
Loans and leases		28,900,000		47,900,000
Held-to-maturity Securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Expected allowance for credit loss		0		0
Fixed-to-Floating Rate Subordinated Notes Due 2030
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt instrument face amount		$ 350,000,000		$ 350,000,000
Interest rate (percentage)		3.375%		3.375%
Shares issued on conversion (in shares)		0.025000		0.025000
Liquidation preference per share (in USD per share)		$ 25		$ 25
Non-Cumulative Perpetual Preferred Stock, Series A
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Dividend rate (percentage)				5.375%
Preferred stock, par value (in dollars per share)		0.01		$ 0.01
Liquidation preference per share (in USD per share)		$ 1,000		$ 1,000
Depositary Shares
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock issued (in shares)	13,800,000
Class A Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock repurchased during period			$ 135,386,000		$ 325,916,000
Class A Common Stock | Repurchases From Ella Ann Holding
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock repurchased during period (in shares)				45,000
Number of shares authorized to be repurchased (in shares)		250,000		250,000
Cumulative effect of adoption of ASC 326
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserve for unfunded commitments							$ 8,900,000
Retained earnings							36,900,000
Allowance for credit loss							(37,900,000)
Allowance for credit loss on debt securities held to maturity							0
Cumulative effect of adoption of ASC 326 | Non-PCD Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss							(56,900,000)
Cumulative effect of adoption of ASC 326 | PCD Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss							$ 19,000,000.0