Accounting Policies and Basis of Presentation Accounting Policies and Basis of Presentation (Recently Adopted Accounting Standards) (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 12, 2020	Jul. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock issued (in shares)			(21,000)		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)			346,000	205,500	695,390	448,500
Stock repurchased during period			$ 126,991,000	$ 89,800,000	$ 286,695,000	$ 190,500,000
Stock repurchased during period (in USD per share)			$ 366.98	$ 436.81	$ 412.23	$ 424.77
Forecast period (years)					2 years
Expected credit losses on SBA-PPP loans			$ 0		$ 0
Reserve for unfunded commitments			13,700,000		13,700,000
Retained earnings			3,651,237,000		3,651,237,000				$ 3,658,197,000
Allowance for credit loss			222,450,000	$ 226,583,000	222,450,000	$ 226,583,000	$ 209,259,000		225,141,000	$ 228,775,000	$ 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			31,887,774,000		31,887,774,000				28,322,780,000
Loans and leases			326,099,000	303,233,000	651,647,000	594,155,000
Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans and leases, not acquired in a transfer accounted for as debt securities			22,156,874,000		22,156,874,000				$ 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			3,084,549,000		3,084,549,000
Loans and leases			19,000,000.0
Held-to-maturity Securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Expected allowance for credit loss			0		0
Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock repurchased during period		$ 47,100,000
Stock repurchased during period (in USD per share)		$ 399.77
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				$ 89,774,000		$ 190,530,000
Class A Common Stock | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock repurchased during period (in shares)		117,700
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